All those arrested or taken in for questioning were Australian citizens of either Somali or Lebanese descent.

Three years ago, a Somali taken in as a refugee and given citizenship returned to his homeland and was killed in fighting in the failed state. Others are believed to have returned to attend al-Shabaab training camps.

The Australian newspaper reported that electronic surveillance suggested the group planned to arm themselves with automatic weapons and storm Holsworthy Barracks, killing as many soldiers as they could before they themselves were killed.

Four years ago, terrorism laws were amended so that prosecutions could be brought against terrorist plotters who may not have fixed on specific targets.