At 5.50am this morning Jacinda Ardern and her partner Clarke Gayford arrived at hospital - and the country has a new temporary leader.

Winston Raymond Peters is now installed as Acting Prime Minister and will be leading the country for the next six weeks.

Ardern has made plain in the lead-up to the birth of her baby that she expects to still have her finger somewhere near the pulse of power, and she will still receive Cabinet and Cabinet committee papers.

Her staff, including chief of staff Mike Munro and officials in the Department of Prime Ministers and the Cabinet Office, will be on hand to support Peters during his stint in the top job.

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He won't be sworn in, and he will remain in his seventh floor Beehive office rather than move into the Prime Minister's digs on the ninth floor.

Ardern and Peters formalised his duties some time ago and a letter from the PM to her deputy spells out exactly what his duties should be in her absence.

The letter, which they crafted together, sets out the day-to-day operational way of working together but with their roles reversed, Ardern said in May.

Ardern said the letter 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 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, who is Foreign Minister, has not scheduled any overseas trips while Acting Prime Minister.

In the unlikely event he may have to travel, or is otherwise unavailable, Labour's deputy leader Kelvin Davis will be Acting Prime Minister.

Ardern has said that if she had to take off more than six weeks, that would be worked through at the time.