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

May 11, 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

THIS IS NOT A PROSPECTUS. YOUR ATTENTION IS DRAWN TO THE IMPORTANT INFORMATION ON PAGE 2.

UPDATE ON APPOINTMENT OF SOUTH32 NON-EXECUTIVE DIRECTORS

BHP Billiton has announced the following developments in relation to the Board of Directors for South32 Limited:

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

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IMPORTANT INFORMATION

This announcement is not a prospectus or listing document in any jurisdiction but constitutes an advertisement in relation to South32 Limited (“South32”) for the purposes of the UK Prospectus Rules.

Any investment decision in relation to South32 should be made only on the basis of the information contained in the Listing Document for the relevant jurisdiction. The “Listing Documents”, which are available, subject to applicable securities laws, on the BHP Billiton website at www.bhpbilliton.com/demerger, comprise a prospectus which has been approved by the UK Listing Authority in connection with the proposed admission of South32’s ordinary shares to the standard listing segment of the Official List of the UK Financial Conduct Authority and to trading on the Main Market for listed securities of London Stock Exchange plc, an information memorandum in connection with South32’s application for the admission of its ordinary shares to listing on the Australian Securities Exchange and a pre-listing statement in connection with South32’s application for the admission of its ordinary shares to listing on the Johannesburg Stock Exchange. Subject to applicable securities laws, shareholders may obtain printed copies of the Listing Document applicable to their jurisdiction free of charge by calling the Shareholder Information Line (details set out in section 3.9 of the Listing Documents).

Nothing in this announcement should be construed as either an offer to sell or a solicitation of an offer to buy or sell BHP Billiton securities or securities in South32 in any jurisdiction, or be treated or relied upon as a recommendation or advice by BHP Billiton.

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Members of the BHP Billiton Group which is headquartered in Australia

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.