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

October 28, 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

BHP Billiton Plc – Director Declaration

In accordance with UK Listing Rule 9.6.14, BHP Billiton Plc advises that Malcolm Broomhead, Director of BHP Billiton Plc and BHP Billiton Limited, has been appointed to the Board of Orica Limited with effect from 1 December 2015 and as Chairman of the Board of Orica Limited with effect from 1 January 2016.

BHP Billiton Plc Registration number 3196209

Registered in England and Wales

Registered Office: Neathouse Place London SW1V 1LH United Kingdom

A member of the BHP Billiton Group which is headquartered in Australia

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.