Prime Minister Jacinda Ardern has tested positive for Covid-19, her office has announced.

She has been in isolation since Sunday 8 May when her partner Clarke Gayford tested positive.

The Prime Minister has been symptomatic since Friday evening, returning a weak positive last night and a clear positive this morning on a RAT test. At this stage her symptoms are moderate.

Ardern's daughter Neve tested positive on Wednesday, the PM said in an Instagram post.

"Despite best efforts, unfortunately I've joined the rest of my family and have tested positive for Covid-19," she said.

"To anyone else out there, isolating or dealing with Covid, I hope you take good care of yourselves!"

The Prime Minister will be required to isolate until the morning of Saturday 21 May. She will not be in Parliament this week for the release of the Government's Emissions Reduction Plan this coming Monday or for the Budget on Thursday. Travel arrangements for her trade mission to the United States are unaffected at this stage.

The Prime Minister will undertake what duties she can remotely this week, but Deputy Prime Minister Grant Robertson will take the post-cabinet press conference on Monday 16 May.

"This is a milestone week for the Government and I'm gutted I can't be there for it," Ardern said.

"Our emissions reduction plan sets the path to achieve our carbon zero goal and the budget addresses the long-term future and security of New Zealand's health system.

"But as I said earlier in the week isolating with Covid-19 is a very kiwi experience this year and my family is no different."