SYDNEY — Australia's terror alert level was raised to "high" by the Australian Security Intelligence Organisation (ASIO) on Friday.

The move, which had been expected for the last week, was announced by Prime Minister Tony Abbott at a press conference in Melbourne.

ASIO independently determines the threat level.

"The advice is not based on knowledge of a specific attack plan but rather a body of evidence that points to the increased likelihood of a terrorist attack in Australia," the government said in a statement.

"Security and intelligence agencies are concerned about the increasing number of Australians working with, connected to, or inspired by terrorist groups such as ISIL, Jabhat al-Nusrah, and al-Qaeda," it said. "The threat they pose has been increasing for more than a year."

The alert level has been on "medium" since the new four-tier alert system was introduced in 2003, which indicated a terror attack "could occur."

The new level of "high" means a terror attack on Australian soil is "likely." The only level higher is "extreme" which alerts to an imminent attack, or one that has already occurred.

Australians have been advised by Abbott to go about their business as usual. He said there will be increased security at airports, ports, public buildings and major events such as the AFL grand final.

Acting AFP commissioner Andrew Colvin asked the public for "increased vigilance" during this time.

"The message that we wish to pass on this afternoon is a simple one. We want people to be more aware, but we also want them to be reassured," he said.

There will be an increased police presence at upcoming events - however there are no specific threats against locations or events. — Campbell Newman (@theqldpremier) September 12, 2014

Abbott said the government had "no specific intelligence" of a plot for a terror attack in Australia.

"What we do have is intelligence that there are people with the intent and the capability to mount attacks," he told the media.

“Nothing that I have said has anything to with religion. This is about crime, potential crime and combating crime.”

He continued: "We do know that people coming back from the Middle East, militarised and brutalised ... do pose a significant threat to our community. I am confident that our agencies are smarter than terrorists ... and will remain one step ahead of their malice."

Australia's chief spy David Irvine said the amount of Australians joining the fight of the Islamic State has contributed to the raised terror alert level.

There are 60 Australians fighting for ISIS, with up to 100 Australians involved in financing and recruiting for the organisation locally.

"The rhetoric that is now coming out of the Middle East, which is encouraging Australians to take violent action and therefore increases the possibility of attacks here," he said.

"So we see an increase particularly in intent, I won't go into all of the details but I think it is important that we raise awareness partly by raising the threat level and to assure people that the authorities both law enforcement and the security intelligence service are very concerned about this and are watching it with great care."

Australian Opposition Leader Bill Shorten also reiterated Abbott's comments, saying he was briefed by the Prime Minister on this national security issue and they are partners when it comes to matters of national security.

Shorten said it was imperative the ISIS be defeated for the sake of Australia's national interest.

"This is not a distant crisis for us," Shorten said at a press conference. "This raised threat level has been undertaken by ASIO and our intelligence agencies. There is intelligence that there is an increased likelihood of an attack in Australia."

On Wednesday, Australia's Immigration Minister Scott Morrison warned of the local threat from Syria and Iraq.

"We know there is a very real threat by the death cult that is operating over in Iraq and Syria and we know that that threat is not just over in the Middle East," Morrison told the National Press Club on Wednesday.