Taiwan was one of the first places to close borders to China during the early stages of the pandemic, enacting a plan that has kept the population of 24 million to just 439 cases and six deaths.

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However, Taiwan's involvement in the WHO is fiercely political, becoming the latest proxy in the geopolitical fight between the United States and China.

In recent weeks, American President Donald Trump reversed earlier praise of China's handling of COVID-19 to lead calls for an investigation into the virus' origins and for the return of Taiwan to the WHO.

Allies Australia and Canada are among supporters of the USA's position, but until this week New Zealand had declined to offer full-throated support.

Prime Minister Jacinda Ardern sidestepped questions on the issue, saying on Tuesday she had been too busy on the local response to consider the matter.

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Earlier on Thursday, China's Ambassador in Wellington, Wu Xi, wrote to Ardern's government in an attempt to curtail Kiwi support.

"There is only one China in the world, and Taiwan is a part of China," Wu said.

"The world is at a critical stage in fighting against the pandemic.

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"However, the Democratic Progressive Party authority in Taiwan region has been making reckless political maneuvers and trying to hype up Taiwan's participation in the WHO and the World Health Assembly.

"Their real intention is to seek 'independence' by making use of the pandemic. We are firmly opposed to that."

Peters, the leader of minority coalition partner NZ First, said in a further statement it should not be seen as a "political or geo-strategic decision".

"New Zealand has a mature relationship with China that brings considerable benefit to both our peoples," he said.

"This decision is entirely consistent with our one-China policy, which remains firm. Our decision is not about politics, it is about global public health, which impacts the health of New Zealanders."

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Peters said previous discussions with Chinese Foreign Minister Wang Yi gave him confidence there would be no pushback to New Zealand's decision.

"My response to the Chinese Ambassador is to listen to her master, Wang Yi, back in Beijing, who has assured me that China doesn't act behave that way or in any way, the way that she might be suggesting,"he said.

"I trust him and I trust their administration to keep their word."

AAP