Jacinda Ardern has defended making an air force Boeing 757 take an extra trip to the Pacific island of Nauru so she can minimise time away from her infant daughter.

The New Zealand prime minister has asked that the plane fly back from the Pacific Islands Forum to pick her up, one day after her deputy departed for the three-day meeting.

The cost to the taxpayer of the extra trip – estimated at between NZ$50,000 to NZ$100,000 (£25,000 to £50,000) – has been criticised.

Ardern is still breastfeeding her 11-week-old daughter, Neve, who is too young to have the immunisations required to visit Nauru. The prime minister decided that while she could not miss the meeting, she also could not leave Neve for the full three days.

After being questioned about whether she thought the one-day visit was a good use of taxpayer money, Ardern told reporters that she weighed the decision carefully. “The other option was for me not to attend at all but, given the importance that we place on the relationships with the Pacific Islands in the reset, that equally didn’t feel like an option.” The “reset” is a reference to Ardern’s foreign policy in the Pacific.

“When weighing up the logistics I asked officials to check the extra costs I would be imposing on the Crown if I were travelling separately. On balance I decided it was worth me travelling for the full day on the Wednesday to fulfil my obligations as prime minister.”

The finance minister, Grant Robertson, said the government allocated the money for such trips each year. “So actually there is no additional money at all being spent there, it’s simply making use of that 757.”

Others have criticised the decision. AM Show host Duncan Garner said he was “loath to criticise a parent for wanting to be with their newborn … Been there, done that, understand it totally. But just pull out of the meeting.”

Garner said Ardern should have instead let the deputy prime minister and foreign minister, Winston Peters, go by himself. “He’s more than capable of holding court with all the local leaders over a drink at happy hour. He can then come back and brief you, really simple. And better for the environment as well, not two flights.”

Barry Soper, writing in the New Zealand Herald, pointed out that the new Australian prime minister, Scott Morrison, would not be at the meeting. “If Ardern decided not to go because of baby Neve, surely the family focused Pacific leaders, more than any others, would have understood.”

Ardern said the situation was unusual as her daughter was so young. “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 later in September for the UN general assembly.