She said Davis told her before the election that he didn't want to be the Deputy Prime Minister.
"I asked him to wait until after the election to give it some thought and consideration – but more recently he has continued to hold that position."
Davis said he wanted to stay on as deputy leader so he could be a mentor for some of the newer MPs in what he described as a "very big caucus".
"I see myself as supporting the wider caucus."
He said that from the outset, Ardern told him that this was his decision and his alone.
He said the main focus of him being in politics was to make a difference for Māori – "that's what I will continue to do".
Davis' decision might come as a surprise to some, given Ardern has previously said that Deputy Prime Minister would be the party's deputy leader.
That role was vacated by NZ First leader Winston Peters after his party failed to reach the 5 per cent threshold at the election.
That means he, and his MPs, are out of Parliament and Ardern's Cabinet.
Labour's caucus is now meeting to decide on the shape of the Cabinet.
There will be a confirmation vote on the party's leadership, which Ardern is certain to win.
Given she, and senior MPs, have thrown their support behind Davis to stay on as Labour's deputy, he is also almost certain to stay in the job.
In terms of who will be the Deputy Prime Minister, Robertson is seen as the top contender.
It would be seen as a shock if he is not appointed to the role when Ardern makes her announcement at 1pm.
But Robertson would not say if he wanted to job when speaking to media this morning.
Meanwhile, the caucus will also be voting on which MPs get what Ministerial portfolios.
There are six empty seats at the Cabinet table: those vacated by the four NZ First ministers, and the unfilled vacancies of former Labour Party ministers Iain Lees-Galloway and Clare Curran.
The Labour caucus elects the members of Cabinet – "we will work through every ministers' name in that room", Ardern said.
"I think it's important that the team has an overall view of the proposed team and have the ability to endorse that."
Anyone is able to nominate, and there will be a vote "if required". Ardern herself will make "suggestions" on who should be in what job.
But the process Ardern uses means there is unlikely to be any surprises when the caucus votes.
"I do spend the better part of a week in talks with all of our members, working through their expectations, my expectations and then I spend a bit of time socialising the decision.
"What will be presented to the team, many of them will be familiar with because we have worked it through over a period of time."
