Prime Minister Jacinda Ardern has released details of how the running of the country works when she goes on maternity leave and hands the rein to deputy Winston Peters. 

In effect, it will be no different to any other time the prime minister is out of the country which automatically renders Peters acting prime minister. But the extended length of time - six weeks Ardern was hoping to take off after giving birth to her first child - heightened the scrutiny of the situation.

So in a two page letter, released by Ardern's office, the two have sought to work out a clear job description that would see Peters take care of the day-to-day running of New Zealand, while Ardern remained just a briefing away.

"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.

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

Those functions included chairing Cabinet and the Cabinet committees that were usually chaired by the prime minister, overseeing the Government's policy programme, engaging with and directing officials from the Department of the Prime Minister and Cabinet, 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. 

ROBERT KITCHIN/STUFF

Deputy Prime Minister Winston Peters will step into the breach for six weeks when Prime Minister Jacinda Ardern goes on maternity leave.

Both Ardern and Peters rejected any notion that the letter was a "behaviour" contract, to keep Peters in check. The letter was set out in close collaboration between them, and collated rules that were already explained in the Cabinet manual. 

"As I say, this is already set out in the Cabinet manual. There is already guidance on how this operates, we've already undertaken these kinds of situations in the past. 

"The only thing that's different is the length of time. There has been a lot of questions, so for the sake of clarity we've been quite happy to put out what is our usual way of functioning in the public domain," said Ardern.

Peters said he and Ardern had worked closely and consistently on a range of issues over the course of the Government, and this was no different.

"Can I just say the prime minister and I crafted this letter - we've done it together - and in the last six months that's exactly what we've done. Not seven days a week, but we've worked co-operatively. 

"Anyone that says otherwise is, to use [former prime minister sir] Geoffrey Palmer's favourite phrase, is spreading rumour with malice."

He also had a parting shot to journalists asking whether the existence of a letter of expectations meant he was being put on notice: "If you want to be relevant, don't be extremist." 

Peters would not be moving to the ninth floor of the Beehive. Ardern said that seemed like an "unnecessary inconvenience" for him. However her staff, including chief of staff Mike Munro would be available to assist Peters in his prime ministerial duties.

Should Peters' role as foreign affairs minister require him to take any unscheduled or emergency travel, then Labour deputy leader Kelvin Davis would step in to the role, as he had previously done on two occasions when both Peters and Ardern were out of the country on official business.

But Peters was expecting to remain in the country for the entire duration of Ardern's maternity leave. 

Ardern was planning to work right up to her due date on June 17.

 - Stuff