Prime Minister Jacinda Ardern has defended her plan to travel to the Pacific Islands Forum separately from the rest of her contingent, saying it wouldn't add much cost.

The main New Zealand delegation, including Foreign Affairs Minister Winston Peters and a media contingent, arrived in Nauru for the forum on Monday.

Ardern will be travelling to the Micronesia island on Wednesday in the same Air Force plane, which will have to return to New Zealand to pick her up.

HENRY COOKE/STUFF Jacinda Ardern in Tonga earlier this year. Ardern is defending the fact she is flying to Nauru separately to the main New Zealand contingent.

The prime minister could not go for the longer period as her 11-week-old daughter, Neve, is too young to be given the appropriate vaccinations for the trip and, thus, could not travel with her.

Ardern said she had weighed up whether or not to go and had asked about how much extra it might cost. She was told the extra cost would not be exorbitant and the plane could not stay on Nauru anyway, as there was not enough room on the runway to store it.

"Weighing up the logistics around travel I asked officials to check what the extra costs I would be imposing on the crown would be if I were to travel separately," Ardern said.

"They assured me that because of the 757 not being able to remain on Nauru anyway, but having to leave the island so other planes could come in and depart - and also the fact that if it wasn't flying there it would be taking up an hour somewhere else anyway, then on balance I decided it was worth me travelling."

She made the comments after Newstalk ZB political editor Barry Soper estimated on Monday morning the extra fuel costs would be about $50,000.

Ardern said the plane's flying hours were budgeted for anyway and would likely be flown for "training hours" if not used for the five-and-a-half-hour trip back to New Zealand.

The original plan was to send the plane to the Marshall Islands an hour's flight away to get it off Nauru.

Ardern noted that the incident was somewhat unique while her daughter was so young and she didn't anticipate it happening again.

"This is a unique set of circumstances, I don't anticipate being in the situation again."

She is intending to take Neve with her on a trip to New York this month for the United Nations General Assembly meeting.

Ardern will make a series of announcements on Wednesday and attend a leaders' retreat at the forum.