UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16

UNDER THE SECURITIES EXCHANGE ACT OF 1934

January 21, 2015

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:  x Form 20-F    ¨ Form 40-F

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):  ¨

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):  ¨

Indicate by check mark whether the registrant by furnishing the information contained in this Form is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934:  ¨ Yes    x No

If “Yes” is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b): n/a

NEWS RELEASE

BHP BILLITON OPERATIONAL REVIEW

FOR THE HALF YEAR ENDED 31 DECEMBER 2014

BHP Billiton Chief Executive Officer, Andrew Mackenzie, said: “Our operational performance over the last six months has been strong. We are reducing costs and improving both operating and capital productivity across the Group faster than originally planned. These improvements will help mitigate some of the impact of lower commodity prices and we remain alert to opportunities to further increase free cash flow.

“In Petroleum, we have moved quickly in response to lower prices and will reduce the number of rigs we operate in our Onshore US business by approximately 40 per cent by the end of this financial year. The revised drilling program will benefit from significant improvements in drilling and completions efficiency. Our ongoing shale investment program will remain focused on our liquids-rich Black Hawk acreage. However, we will keep this activity under review and make further changes if we believe deferring development will create more value than near-term production.

“We continue to believe that our planned demerger will help support further improvements in operating performance in both the core BHP Billiton and South32 assets. Within BHP Billiton, it would allow us to identify and deploy best practice across our assets more quickly and simplify our organisation to reduce overheads. We are making good progress towards securing the approvals we require to put the proposal to a shareholder vote in May and remain on track to complete the process before the end of the financial year.”

Operational performance summary

Production for the December 2014 half year and quarter is summarised in the table below.

Production guidance for the 2015 financial year is summarised in the table below.

Major development projects

The Escondida Oxide Leach Area Project was successfully completed during the December 2014 quarter and the BMA Hay Point Stage Three Expansion project loaded first coal on 12 January 2015, both on revised schedule and budget. The Escondida Oxide Leach Area Project will not be reported in future Operational Reviews.

At the end of the December 2014 half year, BHP Billiton had seven major projects under development with a combined budget of US$13.5 billion.

Corporate update

On 8 December 2014, BHP Billiton announced that the new company it intends to create through its proposed demerger will be called South32. A final Board decision on the proposed demerger will be made once all necessary third party approvals are secured on satisfactory terms. On this basis, BHP Billiton expects to release all shareholder documentation with full details of the proposed demerger in mid-March 2015, with a shareholder vote taking place in early May 2015. The demerger remains on track to be completed in the first half of the 2015 calendar year.

BHP Billiton expects Underlying attributable profit in the December 2014 half year to include impairment charges in the range of approximately US$200 million to US$250 million recognised as a result of the divestment of conventional petroleum assets in North Louisiana and unconventional gas assets in the Pecos field in the Permian.

The Minerals Resource Rent Tax (MRRT) in Australia has been repealed and was applicable until 30 September 2014. As a result, the MRRT deferred tax asset carried by the Group was derecognised and an income tax charge of US$809 million will be reported as an exceptional item in the December 2014 half year. The Group’s adjusted effective tax rate⁽⁴⁾ is expected to remain in the range of 30 per cent to 34 per cent in the December 2014 half year.

On 12 November 2014, BHP Billiton announced that the review of its Nickel West business was complete and the preferred option, the sale of the business, was not achieved on an acceptable basis. As a result of operational decisions made subsequent to the conclusion of this process, an impairment charge in the range of US$200 million to US$350 million (after tax expense) will be recognised as an exceptional item in the December 2014 half year. At this time, Nickel West remains in the BHP Billiton portfolio and the Company continues to operate the business to maximise production, reduce operating costs and increase free cash flow.

This guidance will be updated should material information or events arise as the Company finalises its financial statements.

Marketing update

The average realised prices achieved for our major commodities are summarised in the table below. Iron ore shipments, on average, were linked to the index price for the month of shipment, with price differentials reflecting product quality. The majority of metallurgical coal and energy coal exports were linked to the index price for the month of shipment or sold on the spot market, with price differentials reflecting product quality.

At 31 December 2014, the Group had 322 kt of outstanding copper sales that were revalued at a weighted average price of US$2.87 per pound. The final price of these sales will be determined over the remainder of the 2015 financial year. In addition, 350 kt of copper sales from the 2014 financial year were subject to a finalisation adjustment in the current period. The provisional pricing and finalisation adjustments will decrease earnings before interest and tax by US$210 million in the December 2014 half year (December 2013 half year: US$196 million increase).

Petroleum and Potash

Production

Total petroleum production – Total petroleum production increased by nine per cent in the December 2014 half year to a record 131.0 MMboe. Guidance for the 2015 financial year remains unchanged at 255 MMboe.

Crude oil, condensate and natural gas liquids – Crude oil, condensate and natural gas liquids production increased by 24 per cent in the December 2014 half year to 62.1 MMboe.

Onshore US liquids volumes rose by 71 per cent in the December 2014 half year to a record 24.4 MMboe. This strong performance was underpinned by continued momentum in the Black Hawk and Permian where liquids production increased by 81 per cent and 107 per cent, respectively.

In our Conventional business, liquids production at Pyrenees and Atlantis increased by 34 per cent and 12 per cent, respectively, supported by strong uptime performance and the completion of new production wells in the second half of the 2014 financial year.

Natural gas – Natural gas production declined by two per cent in the December 2014 half year to 413 bcf.

Strong uptime performance at North West Shelf and Macedon partially offset lower seasonal demand at Bass Strait and the divestment of Liverpool Bay which was completed in the 2014 financial year.

Projects

Onshore US development activity

Onshore US drilling and development expenditure totalled US$1.9 billion in the December 2014 half year. In response to weaker prices, the Company will reduce its operated rig count from 26 at period end to 16 by the end of the 2015 financial year. An update to the drilling and development expenditure budget for the 2015 financial year will be provided with the release of our interim results in February 2015.

The majority of the revised drilling program will be focused on our liquids-rich Black Hawk acreage with activity in the Permian and Hawkville limited to the retention of core acreage. The Company’s dry gas development program will be reduced to one operated rig in the Haynesville, with a focus on continued drilling and completions optimisation ahead of full field development.

The reduction in drilling activity will not impact 2015 financial year production guidance and we remain confident that shale liquids volumes will rise by approximately 50 per cent in the period.

As announced in October 2014, we are actively marketing our Fayetteville acreage and will only pursue a divestment if full value can be realised, consistent with our long-term outlook for gas prices.

Capital expenditure includes land acquisition, site preparation, drilling, completions, well site facilities, mid-stream infrastructure and pipelines. The number of wells drilled and completed can vary significantly from period to period based on changes in rig activity and the inventory of wells drilled but not yet completed at period end.

Petroleum exploration

Exploration and appraisal wells drilled during the December 2014 quarter are summarised below.

Petroleum exploration expenditure for the December 2014 half year was US$268 million, of which US$244 million was expensed. Total petroleum exploration expenditure for the 2015 financial year is now forecast to be US$600 million, a 20 per cent reduction from prior guidance. The program will remain focused on the Gulf of Mexico, Western Australia and Trinidad and Tobago.

The seismic acquisition program in Trinidad and Tobago was successfully completed for the seven deep water blocks accessed between 2012 and 2014⁽¹¹⁾. The acquisition for the two blocks awarded in the 2014 deep water bid round is progressing on schedule⁽¹²⁾.

Potash

Copper

Production

Copper – Total copper production⁽¹⁾ decreased by two per cent in the December 2014 half year to 813 kt. Guidance for the 2015 financial year remains unchanged at 1.8 Mt.

Escondida copper production decreased by two per cent in the December 2014 half year to 553 kt. Strong operating performance, including a 14 per cent increase in truck utilisation, resulted in record material mined for the period. This was offset by the impact of water restrictions during the December 2014 quarter, as anticipated, along with two days of industrial action and a power outage throughout Northern Chile during the September 2014 quarter. Production guidance for Escondida remains unchanged at 1.27 Mt of copper for the 2015 financial year.

Pampa Norte copper production increased by 22 per cent in the December 2014 half year to 125 kt as Spence benefited from higher grades and recoveries. Production at Pampa Norte for the 2015 financial year is expected to remain broadly unchanged from the prior year as higher grades and recoveries at Spence continue to offset declining grades and recoveries at Cerro Colorado.

Olympic Dam copper production increased by four per cent in the December 2014 half year to 82 kt. Production in the 2015 financial year is expected to remain broadly unchanged from the prior year with planned smelter maintenance scheduled for the June 2015 quarter.

Antamina copper production declined by 37 per cent in the December 2014 half year to 53 kt as lower grades more than offset record mill throughput. Average copper grades at Antamina for the remainder of the 2015 financial year are expected to remain at a similar level to the December 2014 half year, consistent with the mine plan.

Lead – Lead production increased by six per cent in the December 2014 half year supported by higher mill throughput at Cannington and higher grades at Antamina.

Silver – Silver production decreased by five per cent in the December 2014 half year as higher mill throughput at Antamina and Cannington was more than offset by lower average ore grades at both operations, consistent with the mine plans.

Zinc – Zinc production increased by 20 per cent in the December 2014 half year and primarily reflected higher grades at Antamina, consistent with the mine plan. Cannington achieved record zinc production during the period underpinned by higher grades and increased mill throughput.

Uranium – Uranium production in the December 2014 half year was broadly unchanged from the prior period.

Projects

Iron Ore

Production

Iron ore – Total iron ore production increased by 16 per cent in the December 2014 half year to a record 113 Mt. Guidance for the 2015 financial year remains unchanged at 225 Mt.

Western Australia Iron Ore (WAIO) production increased by 15 per cent in the December 2014 half year to a record 124 Mt (100 per cent basis) as the ramp-up of Jimblebar continued and we improved the availability, utilisation and rate of our integrated supply chain. WAIO also achieved record sales volumes of 126 Mt (100 per cent basis) in the December 2014 half year as our strategy of increasing the percentage of direct to ship ore unlocks further port capacity.

Our WAIO business continues to perform strongly and we have retained guidance of 245 Mt (100 per cent basis) for the 2015 financial year. Further growth in supply chain capacity to 270 Mtpa (100 per cent basis) is expected to be achieved without the need for additional fixed plant investment. Beyond that, the Inner Harbour Debottlenecking and Jimblebar Phase 2 projects⁽¹³⁾ have the potential to increase total capacity to 290 Mtpa (100 per cent basis) by the end of the 2017 financial year at very low capital cost.

Samarco production increased by 29 per cent in the December 2014 half year to a record 14 Mt (100 per cent basis) as the ramp-up of the fourth pellet plant continues to plan.

Coal

Production

Metallurgical coal – Metallurgical coal production increased by 21 per cent in the December 2014 half year to a record 26 Mt. Our metallurgical coal operations continue to perform strongly and we have retained guidance of 47 Mt for the 2015 financial year ahead of the wet season and planned longwall moves at the Crinum, Dendrobium and West Cliff underground mines.

Queensland Coal delivered record production and sales volumes primarily as a result of increased equipment utilisation and the successful ramp-up of the Caval Ridge mine. Record production at Goonyella, Daunia and Poitrel contributed to the strong result.

Illawarra Coal achieved record production of 4.7 Mt in the December 2014 half year as maintenance efficiencies supported higher equipment utilisation rates. The 41 per cent increase from the December 2013 half year also reflects an extended outage at Dendrobium in the prior period.

Energy coal – Energy coal production decreased by three per cent in the December 2014 half year to 36 Mt. Guidance for the 2015 financial year remains unchanged at 73 Mt.

As anticipated, drought conditions constrained production volumes at Cerrejón given the need to manage dust emissions, while Navajo Coal production declined following lower customer demand arising from the closure of three of the five power units at the Four Corners Power Plant. New South Wales Energy Coal production also declined as a result of processing lower yield material during the period and an additional planned wash-plant outage. At South Africa Energy Coal, higher wash-plant utilisation contributed to a 10 per cent increase from the December 2013 half year which was affected by industrial action.

Projects

Aluminium, Manganese and Nickel

Production

Alumina – Alumina production was broadly unchanged in the December 2014 half year at 2.6 Mt and included record production at the Alumar refinery. Worsley achieved record production in the December 2014 quarter as it recovered from a series of calciner outages in the prior period.

Aluminium – Aluminium production decreased by 16 per cent in the December 2014 half year to 517 kt. Lower volumes primarily reflected the suspension of smelter capacity at Alumar and cessation of smelting activities at Bayside during the 2014 financial year and more than offset record production at Mozal in the December 2014 half year.

Manganese ores – Manganese ore production increased by seven per cent in the December 2014 half year to a record 4.6 Mt as an improvement in ore recovery at Mamatwan and an increase in plant availability at Wessels underpinned record production at Hotazel.

Manganese alloys – Manganese alloy production increased by 23 per cent in the December 2014 half year, supported by increased smelter stability and availability at both TEMCO and Metalloys.

Nickel – Nickel production declined by 11 per cent in the December 2014 half year to 70 kt. Lower volumes reflected the closure of the Perseverance underground mine at Nickel West in November 2013 and lower grades and recoveries at Cerro Matoso. Nickel West and Cerro Matoso production guidance for the 2015 financial year remains unchanged at 95 kt and 43 kt, respectively.

As announced on 19 August 2014, in contemplation of the proposed demerger, BHP Billiton and Anglo American have agreed to make certain changes to the agreement which governs their interests in the Manganese business. Subject to obtaining the required approvals for the agreement, expected in the second half of the 2015 financial year, the changes will result in BHP Billiton and Anglo American agreeing to share joint control of the Manganese business.

Minerals exploration

Minerals exploration expenditure for the December 2014 half year was US$154 million, of which US$115 million was expensed. Greenfield minerals exploration is predominantly focused on advancing copper targets within Chile, Peru and the South-West United States.

Variance analysis relates to the relative performance of BHP Billiton and/or its operations during the December 2014 half year compared with the December 2013 half year, unless otherwise noted. Production volumes, sales volumes and capital and exploration expenditure from subsidiaries (which include Escondida, Jimblebar, BHP Billiton Mitsui Coal and our manganese operations) are reported on a 100 per cent basis; production volumes, sales volumes and capital and exploration expenditure from equity accounted investments (which include Antamina, Samarco and Cerrejón) and other operations are reported on a proportionate consolidation basis.

The following footnotes apply to this Operational Review:

The following abbreviations may have been used throughout this report: barrels (bbl); billion cubic feet (bcf); cost and freight (CFR); cost, insurance and freight (CIF); dry metric tonne unit (dmtu); free on board (FOB); grams per tonne (g/t); kilograms per tonne (kg/t); kilometre (km); metre (m); million barrels of oil equivalent (MMboe); million cubic feet per day (MMcf/d); million tonnes (Mt); million tonnes per annum (Mtpa); ounces (oz); pounds (lb); thousand barrels of oil equivalent (Mboe); thousand ounces (koz); thousand standard cubic feet (Mscf); thousand tonnes (kt); thousand tonnes per annum (ktpa); thousand tonnes per day (ktpd); tonnes (t); and wet metric tonnes (wmt).

Further information on BHP Billiton can be found at: www.bhpbilliton.com.

BHP BILLITON PRODUCTION SUMMARY

Throughout this report figures in italics indicate that this figure has been adjusted since it was previously reported.

BHP BILLITON PRODUCTION

Refer footnotes on page 16.