The brigade was raised as a Militia formation in December 1941 in response to Japan's entry into the war. The brigade's headquarters was opened in Sydney, New South Wales, although its constituent units were formed from a number of Australian states. Upon establishment the brigade was raised as an independent unit, and consisted of three infantry battalions – the newly re-formed Victorian-based 39th, the Queensland 49th and the 53rd from New South Wales – which were supported by the 23rd Heavy Anti-Aircraft Battery and the South Australian-based 13th Field Artillery Regiment, which was equipped with First World War vintage 18-pound field guns. The 7th Field Company was also allocated. From the outset, it was formed to garrison Port Moresby, and indeed one of its battalions, the 49th, had been deployed to New Guinea as early as March 1941. Following the attacks on Pearl Harbor and Malaya; however, the decision was made to boost the force around Port Moresby up to a brigade-group sized force and as a result the 30th Brigade was formed. On 3 January 1942, the brigade headquarters and other elements deployed to New Guinea arriving on the troopship RMS "Aquitania", joining the 49th.