On 25 June 1950 , the North Korean Army ( KPA ) crossed the border into South Korea and advanced for the capital Seoul , which fell in less than a week . North Korean forces continued toward the port of Pusan and two days later the United States offered its assistance to South Korea . In response the United Nations Security Council requested members to assist in repelling the North Korean attack . Australia initially contributed P @-@ 51 Mustang fighter @-@ bomber aircraft from No. 77 Squadron RAAF and infantry from the 3rd Battalion , Royal Australian Regiment ( 3 RAR ) , both of which were stationed in Japan as part of the BCOF . In addition , it provided the majority of supply and support personnel to the British Commonwealth Forces Korea . The RAN frigate HMAS Shoalhaven , and the destroyer HMAS Bataan , were also committed . Later , an aircraft carrier strike group aboard HMAS Sydney was added to the force .