New Zealand Prime Minister Jacinda Ardern has called a general election for September, in a vote that will test whether her widespread popularity overseas is matched by support at home.

The centre-left leader on Tuesday announced Kiwis would go to the polls on September 19, two months ahead of the last possible date for the ballot, when she will seek a second three-year term. "I will be asking New Zealanders to continue to support my leadership and the current direction of the government, which is grounded in stability, a strong economy and progress on the long term challenges facing New Zealand," Ardern said.

The 39-year-old's first term won her international fame -- she became a mother while in office and received praise for her sensitive handling of the Christchurch mosques killings and the White Island volcano tragedy.

But while she has been feted overseas, opinion polls show her standing at home has slipped. Opposition leader Simon Bridges has led the centre-right National party into more populist territory, attacking Ardern over a land dispute with Maori groups and attacking her gun buy-back scheme introduced after Christchurch.

Ardern has also come under fire for her party's long-running KiwiBuild scheme, which was designed to make owning a home more affordable by constructing 100,000 homes, but has so far failed to match expectations.

Bridges reacted to the election announcement by pledging to lead a government that "will deliver" on its promises "New Zealanders know we will get things done, whether it’s more money in your pocket, a stronger economy, less tax, building infrastructure and roads or keeping families safer from increasing gang violence," he said.

The most recently published opinion polls, both produced late last year, showed Bridge's National Party ahead but Labour, with its New Zealand First and Green Party allies could muster enough support to remain in power.

The election announcement comes just days before the government is to announce details of a NZ $12 billion (US$7.85 billion) infrastructure spending package designed to stimulate the economy. The New Zealand economy has struggled under low growth, while the cost of living has risen.

Finance Minister Grant Robertson said the infrastructure spend to be released this week will target roads, rail, schools and healthcare projects across the country.  ANZ Bank's latest economic outlook said this would add to an "improved domestic outlook" and it expected the central bank to keep the official cash rate on hold at 1.0 percent for the foreseeable future.

The poll date avoids school holidays and All Blacks matches, which are said to have a bearing on election outcomes.