An IT systems outage blamed for long waits in processing passengers' passports at all Australian international airports has been fixed, but authorities say delays are still expected.

Key points: Passengers in Sydney reported delays of almost two hours to clear immigration in Sydney

Passengers in Sydney reported delays of almost two hours to clear immigration in Sydney The IT problem, which has now been "resolved", hit every international airport in Australia

The IT problem, which has now been "resolved", hit every international airport in Australia Authorities say delays are expected to continue as they clear a passenger backlog

The IT problem, which hit all international airports, caused the SmartGate passport control machines to go down from about 6:00am AEST on Monday.

Passengers posted photos of long queues at Sydney's international terminal — where people were forced to wait for around 90 minutes — and Melbourne's international terminal.

Long delays were also reported at Brisbane.

An Australian Border Force (ABF) spokesperson said the IT system had now been "restored", however ongoing delays were expected into Monday night.

It is not yet clear what caused the incident.

Thousands fill the arrivals hall at Sydney Airport. ( Twitter: David Berger )

"Additional ABF staff have been deployed to process passengers at international airports and to minimise delays in cargo processing where possible," it said in a statement.

"While the addition of staff has seen reduced delays at some airports, passengers are still encouraged to arrive at airports early to allow additional time for processing.

"Cargo processing is continuing, though some delays can be expected as staff work through the backlog."

ABC reporter Francene Norton, who was travelling from Brisbane Airport on Monday morning, said the line was so long it was almost outside.

"I was walking and walking trying to find the end of the queue and I kept walking and walking and there were people lined up everywhere," she said.

"I was virtually out into the car park before I found the end of the queue."

The scene this morning at Melbourne's international airport tells the story. ( Twitter: Michelle Williams )

Former ABC reporter Annabelle Regan, who landed at Sydney Airport on Monday morning, said she could not see the end of the queue she was in.

Celebrity chef Adam Liaw was also caught up in the chaos in Sydney and said there were "thousands of people" stuck in queues.

At Melbourne Airport there were reports of queues more than 200 metres long.

The outages at Brisbane started just after 6:00am, while a Perth Airport spokeswoman said systems had been affected by the outage.

A Qantas spokesperson confirmed many of the airline's outbound international flights at Melbourne and Sydney were delayed by about 30 to 45 minutes due to passengers being stuck in processing queues.

In Brisbane, the line to clear immigration stretches back into the check-in area. ( ABC News: Lexy Hamilton-Smith )

The IT crash has caused some security experts to question Australia's readiness as it prepares to implement of the proposed biometric system and a future with no passports.

ANU National Security College senior research fellow Jacinta Carrol said it was a concern that Australia's "world-leading system" could go down for some time.

"We'll wait to see what the ABF says about the problems that caused this," she said.

"But the concern now would be that if there is an outage and it's out for some time, that you are relying on people [at immigration checkpoints] who might not have the same training as others.

"It will stretch the experience, training and skill level of those staff."