Aust terror alert could go up: ASIO head

Australians could soon be put on a higher terrorism threat alert level.

ASIO director-general David Irvine is seriously considering lifting the level from medium because of the number of people returning from fighting in Iraq and Syria.

Security agencies were aware of more than 20 people in Australia who had fought in the Middle East and posed a threat to national security, he told the ABC on Tuesday.

The threat to Australia was now "a very elevated level of medium".

"I'm certainly contemplating very seriously the notion of lifting it higher because of the numbers of people that we are having to be concerned about here in Australia," he said.

The "medium" threat rating means a terrorist attack could occur in Australia and the "high" level is used when government and agencies believe an attack is likely.

Australia's four-level alert system goes to extreme, when an attack is imminent or has occurred.

In announcing plans to strengthen anti-terrorism laws in August, Prime Minister Tony Abbott stressed the terrorist threat to Australia had not changed.

Australia has been at the "medium" alert level since the four-tier system was introduced in 2003.

Mr Irvine retires at the end of this week.

He hands Australia's top security job to Duncan Lewis, a diplomat, former government national security adviser, defence department secretary and special forces soldier.

Mr Irvine said Australia had been named as a target for several years in publications from al-Qaeda and other terrorist organisations.

"We are certainly aware of people wanting to conduct terrorist attacks in the West and in Australia," he said.

"Whether it's a little bit more or a little bit less than 2001, I think we are facing a persistent threat."