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Police stormed five Sydney properties on Saturday and arrested four men allegedly linked to the plot, with extra security measures extended to all major domestic and global terminals from yesterday.

Four men have been arrested after the NSW Joint Counter Terrorism team conducted raids throughout Sydney suburbs. They are being held under anti-terror legislation that gives investigators a week to bring charges, but requires a magistrate's periodic review and consent.

The Department of Homeland Security said Sunday it is closely monitoring the foiled Australia terrorist plot in which extremists planned an attack by targeting an airplane.

"It'll be alleged that that this was an Islamist extremist, terrorist motivation", the prime minister said.

Prime Minister Malcolm Turnbull said that security has been increased at Sydney Airport since Thursday because of the plot.

Mr Turnbull was later asked why the government hadn't beefed up resources along with stepping up security.

"We're not a gateway airport like Sydney, Melbourne and Brisbane".

"At this time, we don't have a great deal of information on the specific attack, the location, date or time, however, we're investigating information indicating the aviation industry was potentially a target of that attack". The ABC reported waiting time of up to 90 minutes.

The increased security measures also were extended to all major worldwide and domestic terminals around Australia overnight.

Turnbull also said police had seized a "considerable" amount of material as part of the raid.

"Every day, every hour, we are focused on ensuring that our defences against terrorism are stronger than ever".

The national terror threat level remains at "probable".

Qantas and Virgin issued travel alerts to passengers to arrive at the airport two hours before domestic flights and three hours before global flights and to "limit the amount of carry-on and checked baggage".

While they have not been charged, authorities believe the attack was inspired by the Islamic State, Australian Federal Police Commissioner Andrew Colvin said.

But a number of users on social media praised Qantas and Sydney airport workers for getting passengers through quickly.

Virgin said there are no changes to what can and can't be carried on-board the aircraft.