Auckland Airport is blaming ''extreme'' IT network congestion which led to flight delays after some services were knocked out, including baggage check-in.

Passengers faced delays through most of Saturday after intermittent faults to systems including telephone, radio, baggage and Wi-Fi reliant services.

The airport said it was forced to mobilise ''contingencies'' to ensure passenger baggage could be processed and continues work to make sure faults don't happen again.

The network issues were identified at 4am and fixed before midnight on Saturday.

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The problem was tracked to four analogue voice gateway switches on a peripheral network - not the core network.

''Although most travellers were unaware of and weren't affected by it, we acknowledge that there were some flight delays. We apologise for the inconvenience that this may have caused those travellers,'' said Jason Delamore, the airport's general manager marketing and technology.

Delamore said the airport continued to monitor and further investigate the issues experienced to understand why the fault occurred.

''We and other airport partners are taking immediate steps to improve redundancy and resiliency in what is a very large and complex IT network infrastructure.''

The problems hit as the airport had just got through its 10 busiest days of summer, which stretched over a period from December 21 to January 18. Up to 40,000 passengers a day were using the airport on the busiest days.

Around Christmas there was some gate congestion leading to some aircraft being stuck on the tarmac.

Figures out today show Auckland Airport ranks 874 in a league of 1133 airports around the world.

Aviation consultancy OAG released figures for December which show 71.8 per cent of flights arrived or departed within 15 minutes of schedule.

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By number of flights for the month Auckland is ranked 100th busiest with 7523, 1.3 per cent of which were cancelled.

The top 10 airports by number of flights - including Chicago's O'Hare, Atlanta, Los Angeles and Tokyo - all have better on-time performance than Auckland.