Updated May 22, 2017 16:30:16

Photo: The delays at Sydney Airport are expected to continue into the afternoon. (ABC News)

The passport check-in system is back in operation after going down at some Australian international airports, causing flight delays and big queues for passengers in Sydney.

Melbourne and Brisbane international airports were also believed to have been affected.

Melbourne Airport head of communications Grant Smith said the computer failure had affected the Advanced Passenger Processing (APP) system which is used globally.

It is understood the APP system facilitates a more streamlined border clearance process for travellers.

A Qantas spokesman said staff were manually checking-in passengers during the shut-down, causing delays of about half-an-hour.

The delays are expected to continue into the afternoon, even though the system is up and running again.

The software failure affected all flights into and out of Australia, the ABC understands.

SITA, the company that runs the border-control software, said the outage was not related to a cyber attack.

"We experienced a network connectivity issue. This was caused by a major telecom failure in the UK of a top provider to our datacentre," a SITA spokeswoman said.

"In response, we implemented an alternative communications link and resumed services. Our team continues to monitor the situation.

"We can confirm this incident was not due to a cyber attack."

The major airports were asking that anyone flying internationally today contact their airline for an update on individual flights.

At Sydney airport, passengers said they had been waiting about two hours after the passport check-in system failed early this morning.

Frustrated passenger Ada Dong said her airline had not provided enough details about the delays.

"Which line you should be on, which line not to be in — they've got all our personal information to book the ticket how difficult to hand out boarding card to us, took hours, hours, people waiting, have no direction and don't know when they'll be on," she said.

The computer systems were restored about 11:00am.

The Department of Immigration and Border Protection issued a statement saying additional officers had been deployed as required to minimise the impact to passengers.

It said passengers were advised to contact their airline for further information and allow sufficient time to be processed at airports.

Topics: air-transport, immigration, sydney-airport-2020, sydney-2000

First posted May 22, 2017 11:44:45