Australians going to fight with extremist groups in the Middle East are dying as fast as they are arriving, meaning that the total number of fighters has remained stable at about 70, security agency ASIO has revealed.

ASIO deputy director-general Kerri Hartland told a Senate hearing on Thursday that the organisation knew of at least 20 Australians who have now been killed in Syria and Iraq while fighting with groups such as the Islamic State.

High casualty rate: Australians fighting or supporting extremist groups in the Middle East are dying as fast as they are arriving. Credit:AP

"The overall number of Australians currently fighting with or supporting Islamic extremist groups in Syria and Iraq has remained consistent over recent months," Ms Hartland said.

"However, this does not reflect a reduction in the number of Australian travellers. Instead it reflects the relatively high casualty rate for Australians, with the numbers of new arrivals roughly keeping pace with the fatalities."