Ms Ardern, who still breastfeeds her baby, is headed to the Pacific Islands Forum in Nauru for a day on Wednesday.
The air force plane that dropped her deputy Winston Peters there on Monday is being sent back to New Zealand to pick her up.
The round trip will cost taxpayers an estimated NZ$80,000 ($52,873; £41,173).
"I weighed this up. I even went to the extent of checking whether or not I could hitch a ride with Australia. We checked multiple options to try to find alternate ways for me to get there," she told the NZ Herald newspaper on Monday.
"The option was go for a short time or not go at all. If I didn't go, I imagine there would have been equal criticism... damned if I did and damned if I didn't."
She added that she was told the Air Force plane was unable to stay on Nauru in any case, and would have had to park at the Marshall Islands, which is an hour away.
'Proud of her effort'
Ms Arden was not able to go to Nauru on Monday because of the length of time she would have had to spend away her from 11-week-old daughter.
The baby is too young to get the vaccinations required to visit Nauru.
On social media, reactions to the trip were mixed.
"[I'm] proud our leader is making the effort [to take] a day trip to get to Nauru," said one user on Twitter.
But others questioned whether she needed to go at all.
"It's not necessary [for her to attend] if the deputy PM is attending already..." another added.
Ms Ardern said that if she had decided not to attend, that would have made her the first prime minister since 1971 to not attend the Pacific Islands Forum since 1971 outside of an election cycle.
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