The federal government needs to justify why the defence force used radioactive material during training exercises in Sydney, NSW Greens MP Lee Rhiannon says.

The Australian Defence Force (ADF) said it used a "small radioactive training source" during war games across Sydney between April 27 and May 29.

It said the exercises involved the ADF, US forces, Australian Federal Police and NSW police.

The exercises were based on tracking weapons of mass destruction, it said.

"At no time during this exercise was any danger posed to the general public," the ADF said in a statement on Friday.