Prime Minister Jacinda Ardern has set out in a formal letter the duties to be performed by Winston Peters as Acting Prime Minister when she takes six weeks off to have her baby.

Her due date is June 17 and she is scheduled to take six weeks off, but she said she would still be receiving Cabinet and Cabinet committee papers.

Peters, the Deputy Prime Minister and New Zealand First leader, appeared with Ardern at her post cabinet press conference today, and he will run them when she is away.

Ardern and Peters disputed a Newshub suggestion that the letter was a good-behaviour contract.

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Peters: "Can I just say the Prime Minister and I crafted this letter…any anyone that says otherwise, to use Geoffrey Palmer's favoured phrase, is spreading rumour with malice."

Ardern: "This is a letter that simply sets out our usual operational day-to-day way of working anyway.

"The Deputy Prime Minister and myself contact each other frequently when we are not here in Wellington as well. In many ways, it will be as per usual but simply our roles will reverse for a time."

Ardern said she had set out the duties in writing because there had been some public interest in the leave period. It reflected what was already in the cabinet manual.

"As Acting Prime Minister, you will exercise the functions and powers of the Prime Minister, in consultation with me where appropriate, particularly where matters of significant political, strategic or public interest, or national security arise," the letter from Ardern to Peters says.

"I will continue to receive Cabinet and Cabinet committee papers. You will manage the day-to-day business of the Government, including:

• Chairing cabinet and the cabinet committees usually chaired by me (Appointments and Honours Committee, Cabinet Business Committee and Cabinet Priorities Committee). Agendas for the meetings will be managed between my office and the Cabinet Office in the usual way;

• Engaging with and directing officials from the Department of the Prime Minister and Cabinet;

• Overseeing the Government's policy programme;

• Answering questions directed at the Prime Minister in the House and responding to media inquiries, Official Information Act requests and other correspondence; and

• Attending official engagements.

Peters will not be required to be sworn in, Ardern said.

Peters will remain in his offices on the Beehive's seventh floor rather than moving to Ardern's office on the Ninth Floor.

But Ardern's staff, including Chief of Staff Mike Munro will support Peters as Acting Prime Minister as will officials in the Department of Prime Minister and Cabinet and the Cabinet Office.

Peters, who is Foreign Minister, has not scheduled any overseas trips while Acting Prime Minister.

In the unlikely event of him having to take one, then Labour's deputy leader, Kelvin Davis, would be Acting Prime Minister.

Asked what would happen if she needed to take more than the planned six weeks off, Ardern said that would be worked through if the need arose, as would happen now.

"I'm certainly hoping for the best, planning for the worst but expecting everything to be as we've set out."