The head of Australia’s domestic spy agency, David Irvine, says the country’s official terror threat level could be upgraded in the next few days.

ASIO’s director general has told the ABC’s 7:30 that the threat had been building in Australia over the past year and he had an “elevated level of concern”.

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The threat has been at medium since 2003, which means an attack is possible and could occur.

If it is raised to high, it means an attack is likely.

“I would say that at the moment it is at a very elevated level of medium. I’m certainly contemplating very seriously the notion of lifting it higher,” Mr Irvine said.

Mr Irvine is retiring from the job at the end of the 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.

He told 7:30 there were more than 20 people in Australia who had fought in Iraq and Syria and posed a threat to national security.

“Because of the numbers of people we are now having to be concerned about in Australia, because of the influence of Syria and Iraq on young Australians both in terms of going to those places to fight, but also in terms of what they are doing here in Australia with a potential intent to attack,” he said.

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 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.”

AAP, ABC