Robertson told The AM Show that Ardern had made the right decision to attend: "I think if she didn't go to an important meeting like that we'd see a lot of criticism from people in the media as well, so she's probably damned if she did, damned if she didn't.
"She's taken the decision and I think it's the right one."
The Finance Minister said certain allowances had to be made to acknowledge the fact that Ardern was a leader, but also a mother.
"We're just making sure a young mother can go up there and do her job, I think it's a great thing in 2018 that that's happening," he said.
"Jacinda's doing her job as Prime Minister, representing us, and we should be proud of that."
The Prime Minister is still breastfeeding her 11-week-old daughter Neve, who does not have immunity to visit an environment such as that on Nauru.
The round trip comes at a cost of $80,000 on fuel alone.
"I spent quite a lot of time deliberating over whether or not I would attend the Pacific Islands Forum in Nauru. I analysed all of my options," Ardern told reporters yesterday.
Asked whether she thought the cost was a good use of taxpayer money, Ardern said she asked officials and was told the 757 had to leave Nauru, and that it needed to clock up a certain number of flying hours anyway.
"I never had anything that suggested to me that it was such a significant spend that was of concern."
Ardern said she would be raising the issue of refugees and asylum-seekers in detention on Nauru while she was there and still hoped to visit people in detention centres. Peters and officials were scoping possible opportunities for that.
In 2011, Labour was critical of a decision by then Foreign Affairs Minister Murray McCully after he used Air Force aircraft to fly to Vanuatu and back, collecting other government ministers in Samoa, at a cost of $61,000.
"Being flown around in your own aircraft is a 'nice to have' but in these tough times the cost to taxpayers is vastly more than a commercial flight," Labour MP David Shearer said at the time.
