Australia's right-wing terrorism threat has continued to rise with almost half of domestic spy counter-terrorism investigations now dedicated to ideological extremists.
ASIO director-general Mike Burgess told a Senate estimates hearing in Canberra on Tuesday terror threats were complex and challenging.
"Our investigations in ideologically motivated violent extremism, such as racists and nationalists, are approaching 50 per cent of our counter-terrorism onshore caseload," he said.
"This reflects a growing international trend as well as ASIO's decision to allocate more resources to the threat."
He said the increase was concerning and challenging but needed to be viewed in context.
"Religiously motivated violent extremism remains, in the short term, our most serious terrorist threat."
Mr Burgess said battle-hardened foreign fighters could return to the country, while 14 Australians convicted of terrorism offences were due to end prison terms over the next five years.
"Right now ASIO is aware of multiple religiously motivated violent extremists who want to kill Australians. Groups such as ISIL continue to urge attacks," he said.
The threat of a terrorism attack remains probable, with Mr Burgess telling a hearing last month an incident was likely within the next year.
Mr Burgess also warned foreign spies and extremists were using encrypted messages along with other technology to evade authorities.
He said increased investment in the agency would allow ASIO to connect more dots in combating espionage and terrorism.
"Given the volume and complexity of data, we are not searching for a needle in a haystack, we are searching for a needle in a hayfield," the director-general said.
