Dozens of flights in and out of Auckland airport have been delayed or cancelled due to fog.

Auckland Airport said about 40 regional flights were cancelled and 35 been delayed.

It said travellers should check its website for the latest updates, but the fog has now lifted.

MetService said the blanket of fog would clear during the morning, but was likely to linger in Waikato until the afternoon.

Forecaster Tom Adams said once the sun heated up the fog should start to clear quickly in Auckland.

"Places like Hamilton will take a lot longer to clear, not expecting Hamilton to clear until midday - possibly even early afternoon," Mr Adams said.

"So depending on where you're flying to - it may take some time."