New Zealand's opposition leader Todd Muller quit Tuesday after just seven weeks in the role, leaving his party in disarray with an election against Prime Minister Jacinda Ardern looming in two months.

Muller made the surprise announcement in a brief early-morning statement, saying it had "become clear to me that I am not the best person to be leader of the opposition... at this critical time".

"The role has taken a heavy toll on me personally, and on my family, and this has become untenable from a health perspective," he said.

"For that reason I will be stepping down as leader effective immediately."

Muller ousted his predecessor Simon Bridges as leader of the centre-right National Party on May 22 amid polls showing record backing for Ardern ahead of the September 19 election.

The 51-year-old clawed back some support for National, taking it to 38 percent against 50 for Ardern's Labour, but still trailed 13 percent to 54 in the preferred prime minister rating.

Ardern has won praise for her handling of the coronavirus pandemic, with New Zealand containing the virus and experiencing just 22 deaths in a population of five million.

The youthful leader had faced criticism from Muller after personal details of 18 active COVID-19 cases were sent to the media, but the attack backfired when it emerged last week that the leak came from a member of his own party.

Muller said he would "take some time out of the spotlight" with family and intended to remain in parliament.

His deputy Nikki Kaye is seen as a possible successor, along with the more conservative Judith Collins.