A spike in “chatter” detected in a six-fold increase in counter-terrorism operations during the past year has led to an increase in the terror threat.

Security will be beefed up at this weekend’s football finals and “soft targets” like shopping centres as Australia is put on a high terror alert for the first time.

The decision to raise the alert level from medium to high on Friday follows a spike in “chatter” detected in a six-fold increase in counter-terrorism operations during the past year.

While authorities don’t believe a terrorist attack is imminent they have intelligence there are people within the community who have the intent and capability to mount attacks.

Outgoing ASIO boss David Irvine says the heightened security threat could manifest itself in another Bali bombing-style attack.

But the greater likelihood is “lone actors” using firearms at mass gatherings or in public places such as shopping centres.

Security will be increased at airports, ports, military bases, government buildings and large public events such as football finals in Melbourne, Sydney, Perth and Townsville this weekend.

NSW Police intend focusing their efforts on popular landmarks, such as the Sydney Harbour Bridge, and “soft targets” such as packed shopping centres.

“Complacency is our greatest enemy,” NSW police chief Andrew Scipione said.

But acting Acting Australian Federal Police commissioner Andrew Colvin said the increased threat level should not unnecessarily concern people.

“We want them to go about their normal business as they would on any other day,” he said.

ASIO raised the alert level from medium to high on Thursday night, a decision made public by Prime Minister Tony Abbott on Friday.

It is now higher than levels following the September 11 attacks in 2001 and the Bali bombings in 2002.

The decision was based on what the government described as “a body of evidence” that pointed to the increased likelihood of a terrorist attack.

Security and intelligence agencies are concerned about the increasing number of Australians – estimated to be between 50 and 60 – who are fighting with terrorist groups such as ISIL, Jabhat al-Nusrah and al-Qaeda in Syria and Iraq.

They are also concerned about those returning to Australia who have been “militarised” and others known to be supporting and recruiting for the terror groups.

Authorities have recommended the passports of about 50 people – some as young as 17 – be cancelled.

Mr Irvine said other countries expected that Australia not export “criminals and terrorists knowingly”.

The typical demographic of those wanting to join the extremist groups in the Middle East are 17-25 year old males, although it’s known some women have joined up as well.

They range from the unemployed and mentally disturbed to the well-educated and highly-skilled.

Intelligence sources say social media has been a huge driver in attracting Australians to the Middle East battlefields, and has been a big factor in spreading radical ideology.

Mr Abbott said Australians needed to be aware there were people who wished and were preparing to do them harm.

But security, intelligence and police organisations were “smarter than terrorists and would-be terrorists and are one step ahead of them and their malice”.

“The best place for some of those people if they’ve broken our laws is in a maximum security prison.”

Mr Abbott denied the decision to raise the threat level was linked to the government’s likely support of a US-led coalition that will seek to destroy ISIL in Syria and Iraq.

Mr Irvine said ASIO’s decision was based on expert advice provided to it over a long period of time.

Opposition Leader Bill Shorten, who was briefed by the prime minister before the heightened alert level was announced, said Labor would give the government its full support.

“When it comes to fighting terrorism, we are in this together,” he told reporters in Sydney.

Security plans for world leaders visiting Brisbane for the G20 summit in November remain unchanged.

Queensland Premier Campbell Newman denies the event has made Brisbane a target.

“There was always going to be a major security operation for the G20 in Brisbane,” Mr Newman told reporters on Friday.