Air New Zealand is reviving its Covid-19 flexibility policy as customers are being warned of three-hour waits to get through to its call centre.

The national airline is expecting more than half a million customers to fly over the school holiday period that began this weekend.

However, bad weather, high demand and staff illness have been proving difficult.

The airline said it had been hearing from a large number of customers who were falling sick with Covid-19 or other winter illnesses and unable to travel. It was also seeing higher-than-usual employee sickness leading to flight cancellations.

Some flights were cancelled and others were delayed yesterday.

The company has also been continuing to receive an extremely high number of calls, messages and emails, which have been affecting its response time.

Last night the airline said it was responding to increasing levels of illness in the community, and would reintroduce its Covid-19 flexibility policy until 31 July.

It means customers can get credit for their fares if they are no longer able to travel.

The policy allows customers travelling around New Zealand or overseas to cancel their travel if they are unwell and either put their booking into credit or change their flight and have their change fee waived.

The airline's chief customer and sales officer Leanne Geraghty said that with 670,000 passengers booked to travel over the next two weeks, it was inevitable some would either be unable to fly due to illness or they would be called on to care for family members.

"This change will give them the flexibility to change their plans, so they don't miss out on their holidays or being reunited with friends and whānau, and they can still get to where they want to be at a later time."

Customers booked between now and the end of the month can hold the value of their fare in credit for 12 months from the time they requested their credit.

"With sickness and wild winter weather disrupting flights over the past few days, our contact centre team are under an enormous amount of pressure and are doing everything they can to help our customers at this time. So we thank our customers for being patient and understanding while we work through these challenges," Geraghty said.

The airline was encouraging travellers not to call the contact centre if their travel was outside the next 48 hours. Instead, they should use the Manage Booking tab on the airline's app or the website to change their travel plans.

Customers who booked through a travel agent or third-party online agent should contact them directly about making changes to their bookings or credit validity, the airline said.