Australia's elite security agency, the Australian Security and Intelligence Organisation (ASIO), has trebled in size since the September 11 attacks, but such growth is justified, according to the bureau's leader.
ASIO director-general David Irvine said ASIO had to take on more resources to manage a unique terror threat in the wake of the attack on New York.
He said that growth was "certainly justified" but acknowledged there would always be debate about the size and scope of ASIO.
"If you were to ask me what had been the most significant changes in ASIO in the last 10 years, I would say that dealing with the terrorist problem has required us to take on new resources for a start, use new techniques, use new technology," Mr Irvine said in an interview with ABC.
"Legislation has had to have been adjusted to enable us to manage this, at the time, quite different phenomenon - different to anything we faced in the past."
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Mr Irvine rated the threat to Australia from Islamic extremism at the same level it was a decade ago, but says ASIO has increasing concerns about lone-wolf operators such as Norwegian mass murderer Anders Breivik.
"We do have a very small number of people in our midst whose intention are not necessarily all that benign," he said.
Mr Irvine also warned against complacency following the death of al-Qaeda leader Osama bin Laden.
"I don't think we can be complacent and I don't think now, 10 years later, following the death of bin Laden ... I don't think now is yet the time to relax our vigilance.
"His death does symbolise a further step in the weakening of the ability of al-Qaeda from the Pakistan/Afghanistan area to control events, but the threat hasn't diminished because what we've seen is the al-Qaeda ideology and objectives have been taken up by other groups."
