Brigade was formed on 4 April 1940 and was composed of the 2/14th
Battalion from Victoria, the 2/16th from Western Australia, and the
2/27th from South Australia. It was intended that along with the 19th
and 20th Brigades it would form the 7th Australian Division.
By the time
the 7th Division was deployed operationally for the first time in April
1941, however, the 21st was the only original brigade that remained on
its order of battle.
Its sister formations were now the 18th and 25th
Brigades, and would remain so for the rest of the war.