Nauru, Kiribati, Marshall Islands, Palau and the Federated States of Micronesia decided to withdraw from the Forum.
Micronesia's leaders are upset over the appointment of former Cook Islands prime Minister Henry Puna as the Forum's new secretary-general
They say South Pacific countries dishonoured a "gentleman's agreement" under which it was Micronesia's turn.
Jacinda Ardern said the Micronesian leaders' decision was regrettable and saddening.
"We were very saddened by that decision of course, those leaders have given a years lead time to that being enacted and through my exchanges with Minister Mahuta [NZ's Minister of Foreign Affairs], I know that we are going to do what we can to try and encourage the leaders to stay.
"The strength of the Pacific Islands Forum is the wide representation and of course we want to see that remain," she said.
Jacinda Ardern also rejected media reports that New Zealand was backing a particular candidate for the secretary general's position.
"It's just not true, we were looking for a consesus candidate. If anything, we have often been very aware of our role in the Pacific Island Forum, often standing back in some of the debate discussions, trying to be really supportive of that consensus approach, making sure.
We are very mindful of the role we play in the Forum and so I do have to say, I just don't share the view that's been portrayed around our position on candidates," Jacinda Ardern said.