Permanent security upgrades could be imposed at Australian airports to protect travellers from terrorists after authorities foiled an alleged plot to bring down a passenger plane.
Prime Minister Malcolm Turnbull says a a major terrorist plot was foiled on Saturday and has left open the prospect of lasting changes to airport security.
"The heightened security measures at the airport, as everywhere, are under constant review," he told reporters in Perth on Monday, as crowds and queues clogged Sydney airport.
Passenger and luggage screening has been ramped up at all airports after counter-terrorism officers arrested four Sydney men accused of plotting to bring down a plane.
Sydney's domestic terminal 2, which handles Jetstar, Virgin and Tiger flights, appeared to be the worst affected, with passengers spilling out of the terminal doors.
"The queue now stretches outside the terminal and down the entire length of T2 at Sydney airport," tweeted one observer.
Mr Turnbull thanked travellers for their patience but shut down any suggestion of a so-called "terror tax" to better resource airports to deal with heightened security.
"No, no," he said when asked about it.
The government is not revealing any details of the alleged plot, as speculation mounts the Sydney men intended to plant an improvised bomb inside a kitchen grinder or release poisonous gas.
Opposition leader Bill Shorten supported calls for passengers to show identification before they board flights.
"It seems to me at first blush to be a bit sensible that you know who's actually getting on the plane," he told ABC radio on Monday.
Justice Minister Michael Keenan said he didn't want to rule things in or out, "but if there was a requirement to do that to keep the Australian people safe then obviously we would do that".
The AFP has also refused to say if the alleged plot was aimed at a domestic or an international flight.
AFP Commissioner Andrew Colvin said travellers could be very confident in current airport security arrangements.
"What we are doing, what you're seeing at the moment is making sure there is extra vigilance, to make sure we aren't cutting any corners in our security. This is not stagnant though and we are always reviewing what the best arrangements are."
Earlier, Border Protection Minister Peter Dutton said: "It may be that we need to look at the security settings at our airports - in particular our domestic airports - for an ongoing, enduring period."
Airports saw long queues on Monday as travellers underwent extensive checks, including trace testing for explosives.
For travellers, the new normal is a directive to arrive at airports two hours before domestic flights, and three hours before international flights.