Labour leader Andrew Little has stepped down from the role.
He told media poll results were "disturbing" and "very very disappointing".
"It became pretty clear that the story around those polls was building a narrative... I take responsibility for that," he said.
He said he expects Jacinda will be nominated or step up. Little said Ardern will have his full support.
Mr Little said it was his own choice to step down.
This comes after a poll result put Labour on just 24 percent, and Mr Little revealed to media he offered to step down as leader.