Jacinda Ardern calls out Duncan Garner for 'falling victim' to April Fool's Day joke

The Prime Minister has corrected the record after Duncan Garner repeated an April Fool's Day joke claiming Winston Peters would be sworn in as full Prime Minister.

"It is a tough job Duncan - one, I should add, I'm not giving up while I'm on maternity leave," Jacinda Ardern mused on The AM Show on Tuesday morning.

"Even though I understand you may have fallen for an April Fool's and said that someone else would be sworn in.

"There will not be another Prime Minister sworn in in my absence - I'll simply have a fill in for the maternity leave."

But Garner played it off as a joke of his own.

"I wanted to do it because I wanted to see if we were going to be corrected for the third week at the start of an interview with the Prime Minister and yes - you fell for it."

On Thursday, Garner announced Mr Peters would become New Zealand's 41st Prime Minister rather than filling in as acting Prime Minister.

"It's true. It's official," he declared.

"Winston Peters will not be the acting Prime Minister. This has all happened, I'm telling you this for the first time this morning.

"He will actually be sworn in as the country's 41st Prime Minister. He gets his wish after 40 years in the game."

Garner even went on to note Mr Peters' achievements to date.

"He's certainly experienced, he's been deputy Prime Minister before, twice. Acting Prime Minister numerous times, treasurer - that went well.

"Minister of this and that and of course he's been sacked a handful of times as well.

"Bye, Prime Minister - now he's the boss when Prime Minister Jacinda Ardern goes on leave for six weeks.

"What could go wrong? He could just surprise. He's more chill nowadays, he's a statesman. He can be charming. The history books will record this. They will record Winston Raymond Peters officially as the Prime Minister, even if it is for just 42 days.

"Now you know why he went for the Labour Party."

A bemused Amanda Gillies was highly sceptical at the time, noting the date to Garner.

"This is an April true story," he replied.

"I tried to tell him Jacinda," Gillies said on Tuesday.

"He wouldn't believe me."