03/08/2017 7:45 PM AEST | Updated 04/08/2017 8:12 AM AEST

Two Sydney Men Charged Over Alleged Plane Terror Plot

The charges come after four men were arrested on Saturday

Two Sydney men have been charged with terrorism offences on Thursday night over an alleged terror plot to down a plane on the weekend.

The brothers Mahmoud Khayat, 32, and Khaled Mahmoud Khayat, 49 -- who have been in police custody since Saturday -- were both charged by the NSW Joint Counter Terrorism Team on crimes relating to the preparation or planning of a terrorist act.

The maximum possible jail sentence for the offence is life imprisonment.

The charges come after four men were arrested by police in connection to the alleged "Islamic inspired" plot on Saturday and kept in custody by authorities.

Late on Sunday, a magistrate granted the Australian Federal Police (AFP) an additional period of detention in accordance with the Crimes Act, meaning that the men were able to be held for up to seven days without charge.

Abdul Merhi, 50, was later released by police at about 7pm on Tuesday night without being charged and his lawyer promised to "review police action" after his client was found to have no involvement in the alleged plot.

Another man linked to the alleged plan remains in police custody "under specified time provisions" on suspicion of hatching a plan to down a plane using a bomb or noxious gas inside a meat mincer with assistance from Islamic State in Syria.

"Legislation recognises that terrorism investigations are inherently complex and that there can be legitimate reasons for extended periods of detention for suspects in such matters," the statement from the Australian Federal Police said.

Police officers block a small alley where police vans are parked at a home being searched after Australian counter-terrorism police arrested four people in raids late on Saturday across several Sydney suburbs in Australia, July 30, 2017. REUTERS/David Gray

It's understood that the IS-linked plot was uncovered through tip offs from intelligence agencies in the UK and the United States, prompting police to bring forward raids on properties in Sydney suburbs. But the Turnbull government has vehemently denied that it was pushed into the raids by British threats to issue a public travel warning about Australian flights.

National police conducted warranted searches of six locations around Sydney this week, including in Lakemba, Punchbowl, Surry Hills, Wiley Park and Bankstown and the investigation into the alleged terror plot remains ongoing.

Major airports around the country were also affected by the arrests, with long queues and lengthy waits seen in multiple locations after increased security measures were introduced by the Federal government.

The situation was worst at Sydney Airport, where the queue snaked out of the domestic terminal and onto the pavement, and an increased police presence and additional screening of both carry-on and check-in luggage was enacted at all national major airports.

The changes also saw all four major Australian airlines issue warnings to travellers looking to fly on domestic and international flights to leave more time before their scheduled departure.

Qantas, Virgin Airlines, Jetstar and TigerAir have all delivered travel alerts to customers due to the long queues seen at major airports around the country on Monday, saying travellers should arrive two hours before domestic flights, three hours before international flights and also limit any carry-on and checked baggage to reduce security screening time.

"The Australian Government has introduced additional aviation security measures at international and domestic terminals at Australia's major airports," the warnings said.

"Customers can expect to experience an increased level of security scrutiny at the airport so it may take a little longer than usual to get through the process.

"There are no changes to what can and cannot be carried on-board the aircraft."