Isis supporters in Australia are waiting for a "clear signal" from terror cells before unleashing attacks, intelligence analysts have warned.

The warning came as police in Sydney raided several properties after four men of Lebanese descent were arrested for plotting to bring down a commercial airline with toxic gas or an explosive device.

Counter-terror units foiled the attack on Saturday (29 July) and extra security measures have been put in place at major Australian airports.

The terror plot was "advanced", Australian Prime Minister Malcolm Turnbull told ABC radio. He would not specify any further details. Australia's national threat level remains at "probable."

Australian Federal Police (AFP) commissioner Andrew Colvin told a news conference: "In recent days, law enforcement has become aware of information that suggested some people in Sydney were planning to commit a terrorist act using an improvised device."

"We do believe it is Islamic-inspired terrorism. We are investigating information indicating that the aviation industry was potentially a target," he added.

Last month, gunman Yacqub Khayre killed a man and held a woman hostage at an apartment building in Melbourne in an Isis-inspired attack. He was shot dead by police.

Australian police say they have foiled more than a dozen terrorist attacks in recent years, most recently in December when seven men were arrested for planning to bomb landmarks in Melbourne on Christmas Day.

US counter-terror analyst Michael S. Smith II told Australian media: "It very well may be the case that IS is continually engaging with supporters in Australia who are being advised not to execute attacks including simple attacks, such as stabbings or use of cars, until such time as orders are provided."

He added that there have been strong indicators that Isis has released propaganda encouraging terrorism after their supporters have carried out attacks around the world.

"There are ample indicators IS members who are remotely mobilising attacks in the West have sought to time the attacks in a manner that conforms to strategic calculations in part of the group's external operations division," he said.