Newshub can reveal Immigration New Zealand (INZ) and the Security Intelligence Service (SIS) are investigating "several" post-graduate students over concerns their studies here could be used to make weapons of mass destruction.
Some students have already been kicked out of the country over the issue.
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Newshub has learned of one student who's being scrutinised currently - but his lawyer says it's absurd when his client's research is all about improving New Zealand's water quality.
Clean water and a healthy environment is an issue close to the hearts of many Kiwis, but it's raised a red flag for INZ, who believe the student could use knowledge gained here to make weapons of mass destruction.
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"It's absolutely absurd. This person should be at the very front of the queue of people we need in this country," the student's lawyer, Alastair McClymont, told Newshub.
"His employer described him to me as being in the top 0.1 percent of this particular area of expertise in the world."
Newshub has agreed not to identify the student, but he's had to quit his research as a result of INZ's investigation.
"I had to terminate my research and studies. I lost my job because INZ gave me an interim visa with visitor conditions," he told Newshub.
"The long-term stress, pressure and uncertainty was not close to the peaceful life that we expected in New Zealand.
"Even worse, INZ won't give us any timeframe for the investigation, or say exactly what the problem is."
Mr McClymont says as a New Zealander, he finds the situation "extremely depressing".
"We have a brilliant young scientist who wants to settle here and contribute to one of most important parts of our environment being our clean, green environment - yet he's being persecuted and hounded," he explained.
"We are targeted the same as a number of countries for people to come and get knowledge and take that knowledge back to their own countries to develop weapons of mass destruction," INZ assistant general manager Peter Elms said.
Weapons of mass destruction include nuclear weapons, like atomic bombs, and chemical weapons, including the nerve agent Sarin or deadly biological weapons like Anthrax.
"Every assessment that we do has at heart our obligations to meet our international treaties and to protect the New Zealand public," Mr Elms said.
So how many post-graduate students are being investigated? According to Mr Elms, it's a "small number".
"I would estimate that we deal with several cases each year where a real risk exists," he said.
Other agencies help INZ in inquiries like this.
"The SIS would definitely be involved because it is tasked with helping agencies like Immigration with counter proliferation issues," former Green Party foreign affairs spokesperson Keith Locke said.
The SIS told Newshub it has a well-established role in respect of immigration matters, and "it supports the Department of Internal Affairs and Immigration NZ through security screening of individuals applying for citizenship and some visas".
Bill Gates put his mouth on the line a few years ago, drinking water from a sewage purifying plant.
And it's Mr Gates' money, along with another Government research institute, that's helped fund the research that INZ is investigating.
In addition, it was the university - not the student - that suggested the topic which has attracted so much scrutiny.
"The most ludicrous thing is he has to sit and wait for nine months or more for Immigration to make any decisions on his visas," Mr McClymont said.
Mr Elms said that if that takes a long time "we make no apologies for that" - so a stand-off over a suspect student may drag on for some time yet.