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Winston Peters has called for an inquiry into whether a member of the National Party has been working for China's intelligence services.
Earlier this week the Financial Times reported that National Party member Jian Yang spent a decade at elite Chinese military academies and has been under investigation by New Zealand's national intelligence agency.
Mr Peters, whose New Zealand First party is widely expected to form a ruling coalition with either the National or Labour parties, said that New Zealanders should be "very concerned".
"National must act now and a full inquiry is required." Mr Peters said on his official Twitter account.
"There must be proof Dr Yang is not a risk. Meanwhile, Dr Yang must step aside."
Mr Yang who is 55, had lived in China until he was 32, the Financial Times said.
No information about his Chinese education or military background was included in his official biographies in New Zealand or those published when he was an academic at Auckland university, it said.
Mr Yang, who has been a major fundraiser for the National Party and has pushed for closer ties with Beijing, told media that he was the victim of a "smear campaign" and denied passing on any information to China during his time in New Zealand.
"I can understand people can be concerned, because they do not understand Chinese system, but once they understand the system they should be assured this is nothing really you should be concerned about," he said according to a Newshub report.
Prime Minister and National Party leader Bill English said that he was well aware of Mr Yang's background, employment and training in China.
"This is a New Zealand citizen. Dr Jian is a New Zealand citizen. The country is happy to have him as a citizen," Mr English told 1News.
Mr English said he had been aware "from early on" that Dr Yang had had "military training, including military intelligence".
"He's never tried to hide that background," Mr English said.
Meanwhile, Labour leader and contender for Prime Minister Jacinda Ardern declined to comment, saying only it was an "issue for the National Party to respond to".