Authorities in Sydney have seized material that could be used to make an improvised bomb, as investigations continue into an alleged terror plot to bring down a plane.

Key points: "Islamic-inspired" plot involved use of "improvised device", AFP says

"Islamic-inspired" plot involved use of "improvised device", AFP says Four men arrested in Sydney raids

Four men arrested in Sydney raids Extra security measures in place at major airports, some travellers experiencing delays

Prime Minister Malcolm Turnbull this morning confirmed raids carried out in Sydney on Saturday "disrupted" what appeared to be an "elaborate" conspiracy, and said extra security was in place at airports around the country.

Four men were arrested after the raids at five properties in the inner-city suburb of Surry Hills, and the south-western suburbs of Lakemba, Wiley Park and Punchbowl.

The ABC has been told police found items that could be used to make a homemade bomb when they raided the house at Surry Hills.

It is understood that authorities believe the group was intending to smuggle the device onto a plane in order to blow it up.

It is also understood the alleged cell was capable of making an improvised explosive device, but sources say it is unclear whether they would have succeeded in getting it onto a plane.

"We do believe it is Islamic-inspired terrorism. Exactly what is behind this is something that we will need to investigate fully," AFP commissioner Andrew Colvin said.

Police do not yet know what type of plane the group planned to target or what timeframe they had for carrying out the alleged plot.

Mr Turnbull said the "major counter-terrorism operation" was ongoing.

Travellers urged to arrive at airports early

Sorry, this video has expired Five properties were raided overnight

Some travellers experienced delays at airports across the country on Sunday because of the extra security measures.

"Some of the measures will be obvious to the public, some will not be — those travelling should go about their business with confidence," Mr Turnbull said.

"Travellers should arrive at terminals at least two hours before flights to allow ample time for screening. They should limit the amount of carry-on and checked baggage, as this will help to ensure that security screening is efficient."

Officers at the scene of the raid in Surry Hills ( AAP: Sam Mooy )

Virgin Airlines and its budget subsidiary Tigerair released a statement confirming the additional security measures and urging travellers not to be alarmed.

"As the measures place an additional burden on the screening system, it may take a little longer than usual to get through the process," it said.

A Qantas Group spokesperson — representing Qantas and Jetstar — said it was working closely with the Government and airport partners to implement the measures.

"Australia has very strong safeguards in place at its airports; these changes are about making them even stronger," the spokesperson said.

Men can be held without charge for a week

AFP, ASIO and New South Wales Police conduct operations on Cleveland Street, Surry Hills. ( ABC News: Chloe Hart )

Authorities received information about the alleged plot on Wednesday and beefed up security at Sydney Airport on Thursday.

The NSW joint counter-terrorism team decided to move ahead with raids on Saturday evening.

All four men are being held at the Sydney Police Centre in Surry Hills.

The men can be held without charge for around a week.

Sorry, this video has expired A Surry Hills resident left her home saying her relatives had been taken into custody

During that time, police will be searching their homes for documents, computers and electronic devices in order to gather evidence and build a picture of the allegations against the group.

The searches are expected to last for two to three days, and police say there is potential for further arrests.

A woman who told the ABC her son and husband were arrested after her Surry Hills house was raided denied they had any links to terrorism.

"I love Australia," she said as she was accompanied to a waiting car by police.

No charges have been laid against those arrested.

Since 2014, 70 people have been charged as a result of 31 counter-terrorism operations around the country.

Australia's terror threat level remains at probable.