In August 1826 Fremantle was promoted to captain and in 1828 took command of the 26-gun frigate , and sent to claim the west coast of Australia for the United Kingdom. "Challenger" was dispatched by the Admiralty from the Cape of Good Hope on 20 March 1829, anchored in Cockburn Sound on 27 April and landing on Garden Island on 2 May. A week later he hoisted the British flag on the south head of the mouth of the Swan River and took formal possession in the name of His Majesty King George IV of "all that part of New Holland (Australia) which is not included within the territory of New South Wales."