Etihad Airways have confirmed they are assisting Australian authorities with inquiries into an alleged terror plot to bring down a plane leaving Sydney.

Investigations with the airline carrier come after intelligence agencies in Britain tipped off Australian authorities about the alleged conspiracy, which is believed to have involved a bomb hidden in a meat grinder.

The Australian Federal Police is reportedly investigating links between four arrested men, who remain in police custody, and senior Islamic State (ISIS) fighters.

Police continued raids at homes in Surry Hills in Sydney on Tuesday. (AAP) (AAP)

Police continued raids at homes in Surry Hills in Sydney on Tuesday. (AAP) (AAP)

“The Etihad Airways aviation security (AVSEC) team is assisting the Australian Federal Police (AFP) with its investigation and the matter is ongoing,” an Etihad spokesperson told 9NEWS.

“Etihad is complying fully with the enhanced security measures at airports in Australia and monitoring the situation closely. Safety is the airline’s number one priority.”

There are suggestions the four men who remain in police custody after being arrested on Saturday, including Sydney man Khaled Khayat, hold links to ISIS figures overseas and a network of airline staff and may have made a previous attempt to smuggle a homemade bomb onto a flight.

A magistrate approved a move for the men to be held in custody for seven days without charge under anti-terror laws.

It is believed Mr Khayat’s brother is a senior fighter with ISIS in Syria. His son Mahmoud Kayat is also being held by police.

The ABC reports the other two men are related to Ahmed Merhi, who travelled to Syria in 2014 to fight with ISIS. Law enforcement officials told the ABC the detained men may have already made a previous attempt to smuggle their bomb onto an international flight.

9NEWS has been told the arrest phase of the operation developed extremely quickly late last week, when ASIO raised the alarm after what's believed to be a tip off from a foreign intelligence agency.

Investigations are continuing at homes in Lakemba this morning following the arrest of four men. (Network)

Counter Terrorism expert and Research Professor in Global Islamic Politics at Alfred Deakin Institute Greg Barton said GCHQ (Government Communications Headquarters) – the British security agency that monitors signal intelligence – would have picked up something suspicious.

“It comes out of communications with people in Syria… they would have picked something up, passed it onto their domestic agency who is looking for communication with Brits… and would’ve told us,” he told the TODAY Show this morning.

“They’ve got the gear on the ground. They work closely with us… It shows the system works.”

Mr Barton said it appeared as though the men were extremely cautious about what they said outside their close family circles.

“I think it reminds us that this outer circle of people – who aren’t on police radar and we haven’t looked at so much because of lack of resources - we need to go back and look at these people, relatives of people who have been to Syria, frankly,” he said.

“This is another encouragement to speak up if you see something that concerns you.”

(AAP)

He added tougher security measures at Australian airports were the result of an “abundance of caution”, but behind the scenes intelligence was where most of the work was being done.

Police officers raided a unit block in Lakemba linked to Mr Khayat and seized a number of materials including details of a flight number linked to the alleged terror plot.

The property remains sealed off as forensic teams continue searching for evidence.

Properties were also raided in Surry Hills, Wiley Park and two in Punchbowl, and are also ongoing crime scenes.

AAP (AAP)

More warrants are set to be carried out in the coming days.

Travellers at major airports around the country are due for another day of possible delays after Prime Minister Malcom Turnbull yesterday highlighted the government’s commitment to “upgrading and improving our security services”.

More thorough security measures were put in place after Saturday’s arrests in Sydney.

Neil Fergus, security expert and CEO at Intelligent Risks Group, said Australian Federal Police have been pursuing mandatory photo identification for all flights for years.

“The argument was it’s not the person that’s going on the flight, but it’s what they’re carrying. The reality is, is it such a hassle for people to show their ID and for you to have some idea of who is getting on and off flights?” Mr Fergus told the TODAY Show this morning.