New Zealand Prime Minister Jacinda Ardern tested positive for Covid-19 with moderate symptoms, her office said in a statement on Saturday.

She will not be in parliament for the government's emissions reduction plan on Monday and the budget on Thursday, but, "travel arrangements for her trade mission to the United States are unaffected at this stage," the statement said.

Ardern had been symptomatic since Friday evening, returning a weak positive at night and a clear positive on Saturday morning on a rapid antigen test, it said.

She has been in isolation since Sunday, when her partner Clarke Gayford tested positive, it said.

Due to the positive test, Ardern will be required to isolate herself until the morning of May 21, undertaking what duties she can do remotely.

Deputy Prime Minister Grant Robertson will address the media in her place on Monday.

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

"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," she said. "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."

Ardern also said on Saturday that her daughter Neve had tested positive on Wednesday.

"Despite best efforts, unfortunately, I’ve joined the rest of my family and have tested positive for Covid-19," Ardern posted on her official Instagram page.