At 9am 30 June 1921 an active trough was located from the Gulf of Carpentaria to Gabo Island. By 9am 1 July 1 a 1004hPa intense low had quite suddenly developed between Tambo and Dalby, and was rapidly moving in a southeasterly course to the ocean and it formed into a very deep low east of Sydney on 2 July.

From 1st to 6th July practically all sub-tropical rivers and streams flooded to some extent. Almost general dislocation of traffic. Serious inundations in many districts, especially at Goondiwindi where the river was a little higher than in the June flood. There were heavy losses of stock in the Goondiwindi area, 5000 head of sheep being drowned in one district, and 2000 in another.

This event is classified as an East Coast Low.

More about East Coast Lows
Harden Up
Bureau of Meteorology website
The Centre for Australian Weather and Climate Research report