The Chronicle understands the airline needs to use the Bombardiers on routes where it will be servicing with 62-seater Aerospatiale ATR72 planes that were still to arrive.
"We appreciate that Whanganui customers have been enjoying travelling on our larger Q300 aircraft and can reassure you they'll be back flying Whanganui-Auckland soon," she said.
Whanganui Mayor Annette Main said when she heard of the change of planes she contacted Air New Zealand.
"I've been reassured this is a temporary measure based on the need to backfill in areas where they are waiting for 68-seaters to arrive," Ms Main said.
She said she knew people were enjoying the 50-seaters from and to Whanganui.
But she said while this was a temporary change unrelated to passenger issues, she was mindful that passenger numbers needed to be built up to fill the Q300 aircraft.
"That isn't helped by aggressive advertising into Whanganui from Palmerston North airport. But with the better airfare prices we're able to access now, it should be more cost-effective in terms of things like time and car storage costs to fly to Auckland from Whanganui.
"That will help us to lower the risk of changes in the future as well," Ms Main said.
The airline spokeswoman said reverting to the Beechcraft planes would not create any problems in terms of handling bookings. "We've had adequate time to change the aircraft type in our booking system, and therefore available seats, so customers can be assured that we can accommodate their bookings."