Auckland's worst fog in 20 years has disrupted the travel plans of thousands of airline passengers travelling through Auckland Airport as flights are either cancelled or delayed.

And forecasters say there is a risk the fog will stick around to disrupt flights tomorrow morning.

"It may take a few hours to clear up what backlog there still is, but we're unsure of how many passengers are affected or exactly how long it will take," an airport spokesman said.

One News reported tonight that at least 111 departures and 119 arrivals had been affected by the fog. The figures include flight that have been cancelled, diverted or rescheduled.

An Air New Zealand spokeswoman said the fog was "ongoing" and non-jet flights were being cancelled.

She said all jet domestic and international flights which can land in heavy fog thanks to guidance systems are flying normally.

Earlier, Air New Zealand said flights from Auckland to Kaitaia, Blenheim, Rotorua, Palmerston North, Nelson, Napier, Whakatane, Tauranga and the Bay of Islands had been cancelled. Delays on other flights were being posted online.

A Qantas Boeing 737 from Sydney circled over Waiheke Island for 20 minutes and attempted to land but was diverted to Wellington.

Another Qantas flight from Melbourne had been in a holding pattern for 10 minutes.

An Air New Zealand flight from Rarotonga was diverted to Wellington but eventually returned to Auckland and landed.

However, despite the continued risk of fog, it is expected flights will be on schedule tomorrow morning with a south easterly wind likely to "churn the air".

Metservice spokesman Daniel Corbett said: "It should not be as extensive and certainly not as thick but there's bound to be a risk of patchy mist and fog perhaps at the airport and surrounding areas.

"The problem we've had is that there's been no real flow. Imagine a lump of smoke and if you had a good fan blowing at it, it will be gone. But if you've got no wind it just sits there, and that's what happened today.

"As we go through the night, it's started to reform in places. We will see it patchy in the city throughout the night."

- © Fairfax NZ News