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 20, 2016

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

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NEWS RELEASE

BHP BILLITON OPERATIONAL REVIEW

FOR THE HALF YEAR ENDED 31 DECEMBER 2015

BHP Billiton Chief Executive Officer, Andrew Mackenzie, said: “Our operated assets continued to perform well over the last six months. The strong performance of our conventional petroleum assets has offset lower shale volumes following a reduction in investment to preserve the value of our acreage in current market conditions. Increased throughput at Escondida helped mitigate the impact of expected grade decline and better productivity supported production at Queensland Coal. These efforts have allowed us to maintain production guidance for Petroleum, Copper, Coal and Western Australia Iron Ore.

“Commodity prices fell substantially in the first half of the 2016 financial year putting pressure on the whole resources sector. We continue to cut costs and remain focused on safely improving our operational performance to enhance the resilience of our business. In this environment, we are also committed to protecting our strong balance sheet so we have the financial flexibility to manage further volatility and take advantage of the expected recovery in copper and oil over the medium term.”

Summary

Operational performance

Production for the December 2015 half year and quarter and guidance for the 2016 financial year are summarised in the table below.

Major development projects

During the December 2015 quarter, BHP Billiton approved an investment of US$314 million (BHP Billiton share) for the North West Shelf Greater Western Flank-B petroleum project. This follows the delivery of first production from the North West Shelf Greater Western Flank-A project during the December 2015 quarter, which was completed under budget and ahead of schedule. The Flank-A project will not be reported in future Operational Reviews.

At the end of the December 2015 quarter, BHP Billiton had four major projects under development in Petroleum, Copper and Potash, with a combined budget of US$6.9 billion over the life of the projects.

Corporate update

BHP Billiton expects Underlying attributable profit in the December 2015 half year to include additional charges in a range of approximately US$300 million to US$450 million related to: (1) redundancies largely associated with the simplification of our business, rig terminations in Onshore US and closure of the Crinum coal mine; (2) inventory write-downs reflecting significantly weaker commodity prices; and (3) global royalty and taxation matters.

In addition, the Group expects to record a number of exceptional items in the December 2015 half year.

On 15 January 2016, BHP Billiton announced that it expects to recognise an impairment charge of approximately US$4.9 billion post-tax (or approximately US$7.2 billion pre-tax) against the carrying value of its Onshore US assets. The impairment reflects changes to price assumptions, discount rates and development plans. This follows significant volatility and much weaker prices experienced in the oil and gas industry which have more than offset our substantial productivity improvements. This impairment will be reported as an exceptional item in the interim financial results to be released in February 2016.

‘Global taxation matters’ includes potential litigation and tax-related amounts.

On 5 November 2015, the Samarco Mineração S.A (Samarco) iron ore operation in Minas Gerais, Brazil, experienced a breach of the Fundão tailings dam and Santarém water dam. Samarco is continuing to work on quantifying the estimated costs related to the tragedy and, therefore, it is too early to provide an estimate of the financial impact on BHP Billiton. We are continuing to work closely with Samarco and will provide an estimate as soon as we are in a position to do so. BHP Billiton’s 50 per cent interest in Samarco is recorded as an equity accounted investment as one line on the balance sheet. Any charges related to the event are expected to be classified as an exceptional item.

The Group’s adjusted effective tax rate⁽³⁾ is expected to remain in the range of 33 per cent to 37 per cent in the December 2015 half year.

The above guidance will be updated should material information or events arise as the Group 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 2015, the Group had 268 kt of outstanding copper sales that were revalued at a weighted average price of US$2.14 per pound. The final price of these sales will be determined over the remainder of the 2016 financial year. In addition, 363 kt of copper sales from the 2015 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$336 million in the December 2015 half year.

Petroleum

Production

Total petroleum production – Total petroleum production for the December 2015 half year decreased by five per cent to 124.7 MMboe. Guidance for the 2016 financial year remains unchanged at 237 MMboe, as the strong performance by our Conventional business offset lower volumes following a further reduction in Onshore US activity, a third party gas plant outage in the Permian and the successful divestment of our gas business in Pakistan.

Crude oil, condensate and natural gas liquids – Crude oil, condensate and natural gas liquids production for the December 2015 half year decreased by three per cent to 60.5 MMboe.

Onshore US liquids volumes for the December 2015 half year rose by eight per cent to 26.3 MMboe, underpinned by an increase in liquids production from the Black Hawk and Permian of nine per cent and 78 per cent, respectively. However, a reduction in capital spend of approximately 25 per cent contributed to a three per cent decline in liquids production from the Black Hawk and Permian, relative to the September 2015 quarter. Given a further reduction in development activity, annual liquids volumes from these liquids-rich fields are now expected to show a modest decline year on year.

In our Conventional business, a nine per cent decrease in liquids production reflects the impact of industrial action at Bass Strait and natural field decline across the portfolio.

Natural gas – Natural gas production for the December 2015 half year declined by seven per cent to 386 bcf.

The decline primarily reflects lower Onshore US gas volumes as a result of the decision to defer development activity for longer-term value. This was partially offset by higher seasonal demand at Bass Strait.

Projects

Conventional capital expenditure of US$1.5 billion is expected for the 2016 financial year, and remains focused on high-return infill drilling opportunities in the Gulf of Mexico and Western Australia, as well as life extension projects at Bass Strait and North West Shelf.

Onshore US development activity

Onshore US drilling and development expenditure for the December 2015 half year was approximately US$850 million, less than half the December 2014 half year. Our operated rig count remained at seven in the December 2015 quarter, however, we plan a reduction of two additional rigs from the Black Hawk in the March 2016 quarter as we continue to improve drilling efficiency and defer development activity to preserve value. An update to the drilling and development expenditure budget for the 2016 financial year will be provided with the release of our interim results in February 2016.

Petroleum exploration

Exploration and appraisal wells drilled during the December 2015 half year are summarised below.

Our Petroleum exploration is focused in the deepwater Gulf of Mexico, the Caribbean and the Beagle sub-basin off the coast of Western Australia. We are pursuing high-quality oil plays in these three focus areas and continue to invest opportunistically in future growth options. Since August 2015, BHP Billiton has acquired 26 blocks in the Western Gulf of Mexico Lease Sale with a 100 per cent working interest.

Petroleum exploration expenditure for the December 2015 half year was US$321 million, of which US$126 million was expensed. A US$600 million exploration program remains on plan for the 2016 financial year, largely focused on acreage access, seismic data acquisition across our three key focus areas and increased activity in our exploration drilling program.

Copper

Production

Copper – Total copper production for the December 2015 half year decreased by six per cent to 762 kt as continued strong operating performance across the business was offset by grade decline at Escondida, as anticipated. Guidance for the 2016 financial year remains unchanged at 1.5 Mt.

Escondida copper production for the December 2015 half year decreased by 18 per cent to 452 kt as record material mined, underpinned by improvements in truck availability and utilisation, was more than offset by a 25 per cent decline in grade. Increased ore milled during the period was enabled by a drawdown of historically high-cost, lower grade ore inventory. Production guidance for Escondida remains unchanged at approximately 940 kt for the 2016 financial year, as the ramp up of the Organic Growth Project 1 progresses ahead of plan and is expected to reach full capacity during the year. In the medium term, completion of the Escondida Water Supply project and the potential life extension of Los Colorados will enable utilisation of three concentrators to offset grade decline and support a strong recovery in production⁽⁴⁾.

Pampa Norte copper production for the December 2015 half year was broadly unchanged at 126 kt, as record ore milled and higher grade underpinned record production at Spence and offset lower recoveries at Cerro Colorado. Copper production at Cerro Colorado increased by 37 per cent in the December 2015 quarter relative to the September 2015 quarter as mining progressed through a zone of higher-grade ore. The Spence and Cerro Colorado triennial collective bargaining agreements were successfully concluded in December 2015 and January 2016 respectively. Production at Pampa Norte for the 2016 financial year is now expected to be ahead of the prior year despite a short period of industrial action at Cerro Colorado. In the short to medium term, the Spence Recovery Optimisation project will enable the full utilisation of approximately 200 ktpa of tankhouse capacity.

Olympic Dam copper production increased by 37 per cent from the December 2014 half year to a record 112 kt and reflects record ore milled and improved smelter utilisation following planned maintenance in the prior period. Copper grade increased by 35 per cent in the December 2015 quarter in line with the mine plan. Olympic Dam copper production is now on track to exceed 200 kt for the 2016 financial year.

Antamina copper production for the December 2015 half year increased by 36 per cent to 72 kt as higher grades and recoveries were supported by record material mined and milled. Antamina copper production of 136 kt is forecast for the 2016 financial year.

Project

Iron Ore

Production

Iron ore – Total iron ore production for the December 2015 half year increased by four per cent to 118 Mt. Production for the 2016 financial year is now expected to be 237 Mt, four per cent lower than prior guidance, and reflects the suspension of operations at Samarco.

WAIO production increased by six per cent in the December 2015 half year to a record 131 Mt (100 per cent basis), underpinned by the Jimblebar mining hub operating at full capacity and improved ore handling plant utilisation at Newman. This was partially offset by a train derailment and a power outage at the port which reduced volumes in the December 2015 quarter. WAIO guidance for the 2016 financial year remains unchanged at 270 Mt (100 per cent basis) subject to a benign wet season, as continued productivity is expected to offset the one-off operational issues from the December 2015 quarter. The ramp-up of additional capacity at the Jimblebar mining hub and further improvements in the efficiency of the integrated supply chain will deliver an increase in system capacity to 290 Mtpa over time.

Samarco production for the December 2015 half year decreased by 25 per cent to 11 Mt (100 per cent basis). Mining and processing operations at Samarco remain suspended following the breach of the Fundão tailings dam and Santarém water dam on 5 November 2015. Pellets continued to be shipped from port stockpiles during the December 2015 quarter with the final shipment of pellets expected in January 2016.

The sale of BHP Billiton’s 100 per cent interest in its Liberia iron ore project to Cavalla Resources was completed in October 2015.

Coal

Production

Metallurgical coal – Metallurgical coal production for the December 2015 half year decreased by three per cent to 21 Mt. Guidance for the 2016 financial year remains unchanged at 40 Mt.

Queensland Coal production declined in the December 2015 half year as record production at the Blackwater, Daunia, Caval Ridge and South Walker Creek mines was offset by a convergence event at the Broadmeadow mine, which has since resumed normal operations, and completion of longwall mining at the Crinum mine. The Crinum mine will transition into care and maintenance in the March 2016 quarter.

Energy coal – Energy coal production for the December 2015 half year decreased by three per cent to 19 Mt. Guidance for the 2016 financial year remains unchanged at 40 Mt⁽²⁾.

Lower production for the December 2015 half year reflected continued drought conditions at Cerrejón and the impact of heavy rainfall at New South Wales Energy Coal. A 16 per cent increase in Navajo Coal volumes due to higher customer demand was offset by lower customer requirements for our San Juan product.

On 2 July 2015, BHP Billiton announced that the sale agreement for the San Juan Mine to Westmoreland Coal Company (WCC) had been executed. Regulatory approval has been received and the transaction is now expected to be completed in the March 2016 quarter, following agreement with both parties to ensure all closing conditions are met.

Other

Nickel production

Nickel – Nickel West production for the December 2015 half year decreased by 23 per cent to 37.3 kt. Lower volumes reflected planned major outages at the Kalgoorlie smelter and Kwinana refinery during the December 2015 quarter and a reduction in third party ore delivered to the Kambalda concentrator.

Potash project

Minerals exploration

Minerals exploration expenditure for the December 2015 half year was US$89 million, of which US$73 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 2015 half year compared with the December 2014 half year, unless otherwise noted. Production volumes, sales volumes and capital and exploration expenditure from subsidiaries are reported on a 100 per cent basis; production and sales volumes from equity accounted investments 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: bhpbilliton.com

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PRODUCTION SUMMARY

PRODUCTION SUMMARY

PRODUCTION AND SALES REPORT