Australian authorities have arrested four men over plot to bomb a plane over Sydney.

Australian authorities have foiled a terrorist conspiracy to bring down an aeroplane using an improvised explosive device.

Four men are in custody after NSW and Australian federal police officers raided properties in the Sydney suburbs of Surry Hills, Lakemba, Wiley Park and Punchbowl on Saturday afternoon.

Prime Minister Malcolm Turnbull says the operation disrupted “a terrorist plot to bring down an aeroplane”.

“Be assured that our number one priority is to keep you safe,” he told reporters in Sydney on Sunday.

“That is our focus, that is our determination.”

Australian Federal Police Commissioner Andrew Colvin said security agencies became aware “in recent days” of a plan to commit a terrorist act using an improvised device.

"At this stage, we don't have a great deal of information on the specific attack, the location, date or time," he said.

"However, we are investigating information indicating that the aviation industry was potentially a target of that attack."

The plot, Mr Colvin said, was believed to be Islamic State-inspired.

The four men have not yet been charged and have already been in custody for 12 hours. Authorities will need to apply to a magistrate to hold them for longer as the investigation continues.

Mr Colvin said additional searches could take "many more days" and while he confirmed officers found items of interest during the initial raids, he declined to go into detail.

Asked about the capability of the group to carry out the plot, he replied: "We've taken this threat very seriously".

Mr Turnbull said increased security measures have been placed on all Australian airports. #9Newshttps://t.co/3JRHZijjdB

— Nine News Australia (@9NewsAUS) July 29, 2017

Mr Colvin added: "That in itself you should infer that we think this was credible and there was an intention, and there was quite possibly a capability as well."

The alarming plot triggered an "intensifying" of security measures at Sydney Airport on Thursday which has since been extended to all domestic and international departure points.

The prime minister warned passengers should arrive two hours before their flight and minimise carry-on and checked baggage.

"Some of the measures will be obvious to the public, some will not be," Mr Turnbull said.

When asked if the men had any connection to the airports, such as employment, Mr Colvin said the integrity of airport security was not compromised.

Mr Colvin said while terrorists were becoming "very ingenious" about ways to bypass security, he was confident existing measures at airports would have stopped the men from carrying out their plan.

Justice Minister Michael Keenan said it was the 13th alleged conspiracy thwarted since the terror threat level was raised to "probable" in 2014.

Since that time, a total of 70 people have faced charges as a result of 31 operations.

"The primary threat to Australia still remains lone actors," Mr Keenan said.

"But the events overnight remind us that there is still the ability for people to have sophisticated plots and sophisticated attacks remain a real threat."