Screening at all major Australian airports has been ramped up overnight and air travellers have been warned to expect delays following the discovery of a terrorist plot to bring down an aeroplane with a bomb.

Prime Minister Malcolm Turnbull, who said "the threat of terrorism is very real", warned that airport security has been "enhanced and intensified" in light of the operation overnight.

Five properties in Surry Hills, Punchbowl, Wiley Park and Lakemba were raided on Saturday evening.

Four people remain in custody.

Police allege the group of Sydney men were planning to use an "improvised device" to target the aviation industry however no details on a particular target, location, time or date have been uncovered.

Australian Federal Police Commissioner Andrew Colvin said they had no reason to believe airport security had been "compromised" but a heightened state of security has been implemented.

Mr Turnbull urged people to limit carry on and checked in baggage and to arrive much earlier for domestic and international flights.

He said additional measures were put in place at Sydney Airport on Thursday ahead of the operation.

"These and further measures have been extended to all major airports around the country overnight," he said.

"Some of the measures will be obvious to the public, some will not be. Travellers should be prepared for additional scrutiny at screening points."

He said domestic travellers should be prepared to arrive at the airport at least two hours before their flight.

"They should limit the amount of carry on and checked baggage so far as possible, as this will help to ensure that security screening is efficient," he said. 

The national terrorism threat level remains at "probable", unchanged since 2014.

"Our number one priority is to keep you safe," Mr Turnbull said.

Justice Minister Michael Keenan said the plot was the 13th "significant disruption" in Australia and 70 people have been charged as a result of 31 counter terrorism operations around the country since 2014.

"The primary threat to Australia still remains lone actors," he said. "There is still the ability for people to have sophisticated plots and sophisticated attacks still remain a real threat."

Australian Federal Police commissioner Andrew Colvin said early investigations suggest the plot involved the use of an improvised explosive devise to attack the aviation industry.

"Searches are ongoing. This is the start of a very long and protracted investigation," he said.

NSW Police Commissioner Mick Fuller urged Sydney residents to go about their business as normal today but be prepared for disruptions.

"Come into the city, take your trains, go to the shops, you shouldn't be concerned in relation to this," he said. "There is not specific information that means you should sit at home."

About 1pm on Saturday, the Australian Federal Police conducted a number of terrorist raids across Sydney over a suspected bomb plot to bring down a plane.

The raids took place in Surry Hills, Lakemba, Wiley Park and Punchbowl and were not planned but were a rapid response to information about the plot received by police. 

The bomb squad attended the scene of the Surry Hills raid on Cleveland Street, which was closed for several  hours, while the Lakemba raid took place at a home on Sproule Street, not far from Lakemba Station. 

The family who lived in the raided house on Cleveland Street have been described as "perfectly nice and normal people" by a neighbour whose property backs onto theirs.

"We knew them to say hello to and they seemed nice," the woman in her early 30s, who didn't want to be identified, told AAP. An elderly couple lived in the home, the neighbour said, and they had adult children.  

The neighbour came home while the raid was underway and said there were "heaps" of police at the scene.

- The Age