A MAJOR Islamist terror plot to bring down a passenger jet in Australia has been foiled, the country's prime minister has tonight revealed.

Four people were arrested in raids across Sydney in connection with the planned atrocity, Malcolm Turnbull said.

6 Australian police at the scene of one address raided in Sydney during anti-terror operations across the city Credit: EPA

6 Four people were arrested during the five raids carried out in connection with a major Islamist terror plot Credit: EPA

"There has been a major joint counter-terrorism operation to disrupt a terrorist plot to bring down an aeroplane," the Aussie leader told reporters.

It is the 13th significant threat disrupted by police since the terror level was raised in 2014. In that time, five deadly plots have been carried out.

Additional security has been put in place at all Australian domestic and international airports.

It will lead to delays for travellers, who have been advised to arrive for their flights at least two hours before departure as screening is ramped up.

The massive raids involving dozens of officers were coordinated out by the Australian Federal Police, the New South Wales state police and spooks from Australian Security Intelligence Organization.

Local TV channel Seven Network reported that 40 riot squad officers wearing gas masks stormed an inner-Sydney house before an explosives team found a suspicious device.

6 Police and forensics teams in Sydney. Exact details of the planned atrocity are yet to be revealed Credit: EPA

6 Police work in a lane way behind a house raided during terror raids in the inner Sydney suburb of Surrey Hills - one of a number of addresses raided Credit: AFP or licensors

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Law enforcement has been become aware of information that suggested some people in Sydney were planning to commit a terrorist attack using an improvised device Andrew Colvin

A woman led from a raid by police with her head covered told Nine Network Television: "I love Australia."

None of the four suspects arrested in five raids had been charged, police said.

Exact details of the plot have not been released.

Andrew Colvin, Australian Federal Police Commissioner, said: "In recent days, law enforcement has been become aware of information that suggested some people in Sydney were planning to commit a terrorist attack using an improvised device.

"We are investigating information indicating the aviation industry was potentially a target of that attack," he added.

6 TV captured the dramatic raids taking place in the Australian city Credit: 7 News

6 A man who appears to have his head bandaged sits outside an address raided by Aussie cops Credit: 7 News

Justice Minister Michael Keenan said: "The primary threat to Australia still remains lone actors, but the events overnight remind us that there is still the ability for people to have sophisticated plots and sophisticated attacks still remain a real threat.

"In light of this information, it's very important that everyone in Australia remains vigilant."