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ANALYSIS: Deputy Prime Minster Winston Peters will step up to be acting prime minister for the six weeks after Prime Minister Jacinda Ardern has her baby in June.

If you are wondering exactly what an acting prime minister is and can do, we are here to help.

The role of acting prime minister - much like the role of prime minister - doesn't have any basis in the law.

KEVIN STENT/STUFF Winston Peters will be acting Prime Minister for six weeks.

It is instead a creature of convention, codified by the Cabinet Manual, which sets out the entire role in two brief paragraphs.

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If the Prime Minister is unavailable or unable to exercise the statutory or constitutional functions and powers of the office, the Deputy Prime Minister can, if necessary, exercise those powers and functions.

As Acting Prime Minister, the Deputy Prime Minister may exercise other prime ministerial functions and powers, in consultation (where appropriate and practicable) with the Prime Minister.

LAWRENCE SMITH/Stuff Prime Minister Jacinda Ardern announces that she and partner Clarke Gayford are expecting their first child in June.

Since Ardern will be busy but not unreachable, she signalled in her press release that yes, Peters will be in contact with her regularly.

"As is the case when I am overseas, Mr Peters will act as prime minister, working with my office while staying in touch with me. I fully intend to be contactable and available throughout the six week period when needed," Ardern said.

"He's played that role before. When I go overseas he plays that role. It's not unusual."

(It isn't. Even this length of time is not unheard of - Norman Kirk took six weeks off near the end of his life.)

So the convention, if it is not clear enough here, is that any major decisions should go through the real prime minister first. So while the acting PM can make serious decisions their actual power to set the till of the country is quite limited.

There's also the fact that prime ministers in general act in consultation with their Cabinet, not just themselves. So while Peters will have the top job Labour will still easily control Cabinet.

So what are those powers and functions?

The prime minister is the principal adviser to the Queen in New Zealand - represented by the Governor General - which means in practice the power to appoint and dismiss ministers of the crown, and dissolve Parliament in order to call a general election. (It is extremely unlikely Peters will do either of these things).

The other big executive duty involves national security and intelligence.

It is in this area where the most controversial decision was made by an acting prime minister.

When future-PM Geoffrey Palmer was acting PM in Feburary 1985 the US made a request for the nuclear-armed USS Buchanan to visit New Zealand - in direct contravention of our nuclear-free policy.

Palmer believed that Prime Minister David Lange was uncontactable - he was somewhere in the Pacific - so made the decision to turn down the request on his own initiative. As a result the US eventually suspended its ANZUS obligations to New Zealand.

It seems unlikely such a large decision will fall to Peters during his time.

What he will get a chance to do is front the media at press conferences. That should be fun.