Travellers are experiencing long delays and cancellations at Auckland Airport due to a global passenger processing outage.

Airlines have had to revert to manual check-in for international departures for most of Tuesday due to the outage.

Global software company Sita, which runs the passenger processing software, confirmed to the Herald on Tuesday evening services had been restored, but New Zealand and international airports continued to experience delays.

NZME radio producer Max Middleton said around midday crowds had been building up at the international terminal.

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"No flights are being able to be checked in at the moment," he said.

"The airport is chaos and no one seems to know what's going on."

Kate Holdsworth tweeted from the airport "no one can check-in".

"Already queued for an hour and not moved. No one can check in. Not exactly what you expect when you've paid for Business Class," she posted.

Immigration New Zealand (INZ) border and visa operations general manager Nicola Hogg said they were experiencing an outage in their Advance Passenger Processing (APP) system.

This was a global system operated by Sita, a multinational information technology company providing IT and telecommunication services to the air transport industry.

INZ had been advised by Sita it was a global outage, affecting a number of countries, Hogg said.

INZ was monitoring Sita's progress in getting the system up and running and was working with airlines to minimise any disruption to travellers.

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"In the meantime INZ is using manual back-up systems, which could cause some delays to passengers checking in for flights to New Zealand from overseas and when they arrive in New Zealand."

A spokeswoman for Sita confirmed to the Herald about 5pm Tuesday systems had been "fully restored and operations running as normal".

"We regret any impact this incident has had on our customers and passengers.

"Our priority remains, as always, to ensure a stable system so that our customers can conduct their business efficiently and effectively.

"We are currently conducting a full investigation to discover the root cause and though the service is currently restored, our team remains on high alert."

An Auckland Airport spokeswoman said while majority of airlines could manually process passports it was causing some delays at check-in.

"We advise all passengers to contact their airlines for the latest travel information.

"Auckland Airport is working with Immigration NZ, our airline partners and government agencies to provide what support we can to help travellers on their journey."

An Air New Zealand spokeswoman said due to the "global outage of an IT system used by border authorities" the airline had reverted to manual check-in for international departures.

Meanwhile, travellers across the ditch were also experiencing similar issues.

Kiwi Graeme Underwood was at Melbourne International Airport and said it was "chaos".

He was meant to be flying back to Auckland this morning with Air New Zealand, but had been unable to check in.

All airlines were affected, he said, and a staff member told him it was a worldwide check-in issue.

"I am not sure what is happening but the airport is at a standstill."

Air New Zealand staff were handing out meal and drink vouchers to affected customers, he said.

"They are doing a great job accommodating everyone, which makes you think we might be here for a while."

A Melbourne Airport spokeswoman told the Herald there had been a worldwide customs system outage.

Jetstar also issued a travel alert stating an "IT issue" was affecting their airport systems today, resulting in delays and cancellations to some domestic and international flights.

And in Sydney, passengers heading to Sydney Airport have been warned to expect significant delays, with reports the International passenger check-ins are currently down.

According to a spokesperson, the processing system airlines use to check-in passengers is suffering from an outage, meaning passengers should leave plenty of time to get to the airport.