Passengers caught up in a three-hour system outage at international airports in Australia and New Zealand say the check-in queues were the longest they have seen.
Airline staff were forced to manually process customers after the globally-used, border-clearance system known as Advance Passenger Processing went down about 8am on Monday.
Photos uploaded to social media show queues snaking the length of airport foyers in Sydney and Melbourne, as staff worked to clear the backlog.
A Sydney Airport spokeswoman says its system came back on at 10.40am with check-in resuming "as normal". Melbourne's was again online by about 11am with all airlines able to process digital check-ins.
Brisbane Airport was also affected by the shutdown, prompting manual check-ins but no flights were delayed, a spokeswoman told AAP.
Six flights departing from Auckland Airport experienced minor delays when Immigration New Zealand's national system went "down" too, according to the airport's Twitter account.
"Immigration NZ advises the system is now fully functional. Customs arrivals Smartgate will be limited for flights ex Australia that departed during the outage," the New Zealand airport stated.
All airlines at Wellington Airport were able to process check-ins without delays once the "global" issue was amended.
Those flying internationally are advised to contact their airline to check the status of their service.
Anxious customers voiced their concerns about making connecting flights.
"Nobody able to check-in at Sydney Airport. Nobody. I've never seen lines this long in an airport," passenger Chris Walker-Bush tweeted earlier on Monday.
Traveller Andrew Jessup tweeted a photo of the "huge queues" captioned: "Bedlam at Sydney Airport".
All international flights to and from Australia are subject to mandatory APP reporting provided by airlines through the APP system, according to the Department of Immigration and Border Protection.
"This facilitates a more streamlined border clearance process for travellers and enhances the Australian Government's ability to target security concerns," the department's website states.
