The flow of Australians joining jihadist groups has reached "a plateau", the nation's domestic spy chief said Thursday, amid reports the intelligence agency has warned politicians to tone down criticism of Islam, AFP reports.

Duncan Lewis, head of the Australian Security Intelligence Organization (ASIO), said he was "not declaring victory" despite a slowdown in the flow of nationals travelling to fight with militant organisations such as "Islamic State" (ISIS) in the Middle East.

"I don't want to be giving any sense that we are through the worst of this. I don't think that's right," he told the Sydney Morning Herald, adding that some 44 Australians have so far died in Syria.