Air New Zealand is advising passengers travelling through Christchurch Airport to defer travel if they can today to avoid major disruption because of knock-on affects from fog.

Customers booked to travel on both A320 jet and turbo-prop services to or from Christchurch today were advised to watch the Air New Zealand arrivals and departures page or sign up for travel alerts to ensure they are kept up to date with flight information.

Close to 25 arrivals and departures to ports around the country have already been cancelled or delayed although fog is lifting in the mid morning.

Air NZ said it was expecting around 18 weather delays today, mainly on domestic services, due to the fog disrupting early morning flights in/out of Christchurch to/from Wellington and Auckland.

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"Customers who don't have an urgent reason for travel today are strongly encouraged to defer their travel and the airline is offering the following flexibility," the airline said.

Jetstar said it was expecting around 18 weather delays today, mainly on domestic services, due to the fog disrupting early morning flights.

Air New Zealand says if customers deferred travel to a later flight before October 5 they would not face extra costs. If the proposed date of travel is unknown, the value of the ticket may be held in credit for 12 months.

The airline is also warning that its customer contact centre was currently experiencing very high call volumes as a result of the disruption, and customers are advised to avoid calling if possible.

While Christchurch has an instrument landing system to help aircraft fitted with matching equipment and trained crew to land, fog can sometimes be so thick it makes that, and taxi-ing, dangerous.

Most jet aircraft are fitted with systems compatible with the ILS, along with certified crews, but regional turbo-props generally aren't enabled.