Two men are scheduled to face court in Parramatta on Friday morning charged with planning a terrorist act after an alleged Sydney plot to bring down a passenger plane.

A 49-year-old Lakemba man and a 32-year-old Punchbowl man have each been charged with two counts of acting in preparation for or planning a terrorist act, Australian Federal Police said in a statement on Thursday night.

The maximum penalty for the offence is life imprisonment.

Another Surry Hills man remains in police custody and can be detained under special anti-terror laws until the weekend.

A 50-year-old man was released from police custody on Tuesday without charge.

The men were arrested on Saturday in raids by New South Wales counter-terrorism police, with authorities alleging a plot to bring down a passenger plane using an improvised device.

NSW commissioner Mick Fuller foreshadowed the charges earlier on Thursday, describing the evidence against the men as “very strong”.

Prime Minister Malcolm Turnbull echoed the comments.

“Investigations have proceeded very well in terms of uncovering evidence,” he told reporters in Perth on Thursday.

“The police have uncovered very substantial evidence.”

Mr Turnbull announced the threat level for Australian aviation had been downgraded to “possible” on advice from the head of Australia’s spy agency ASIO.

The threat level increased to “probable” when the Sydney plot was discovered.

Mr Turnbull said the additional security measures in place at Sydney airport and other major airports across the country, which caused major delays, are expected to be modified within 24 hours.

He said there still would be enhanced security measures at airports but would be done in a way to reduce delays for travellers.

“You should expect to receive advice from airlines within 24 hours that arrival times before a flight will be restored to those that previously applied — but some airlines differ,” he said.

Police conducted searches across Sydney earlier this week, including in the suburbs of Lakemba, Punchbowl, Surry Hills, Wiley Park and Bankstown.

– with ABC, AAP