The Prime Minister says she thought of not going to the Pacific Islands Forum on Nauru this week, because of baby concerns.

As it is, the Airforce's 757 jet will travel back from Nauru, at a fuel cost around about $50,000, to take Ardern to the tiny island state on Wednesday for a day.

Ardern says she decided it was important for her to be there.

"I analysed all of my options. I'm not in a position at this stage of Neve being just 11 weeks old to have travelled with her as she would not have all the immunisations required to travel with me."

The media and Foreign Minister Winston Peters are already on the island.

Ardern says that these are a "unique set of circumstances".

"There won't be many occasions that have these set of circumstances. It was something I gave a lot of thought to. The other option was not attending, and that didn't feel right to me either."

Ardern who wasn't aware of the cost but was assured it wasn't too much.

"I've never been had it broken down for me in writing, but I did ask the question verbally because I was weighing up all of these issues."

She says she was deliberating this issue while on maternity leave.