US Secretary of State Pompeo said Trump administration 'fully prepared' to counter Iran's effort to circumvent hurdles.

Immigration New Zealand say Donald Trump's hard line on Iran hasn't led to a crackdown on Iranian students here.

But Iranian post-graduate students tell a different story, saying it's getting harder to secure visas to study here, and they blame America's anti-Iran rhetoric.

INZ, however, say there's been no change to a 12-year policy of extra scrutiny of students from countries with programmes to develop weapons of mass destruction.

That means post graduate students with study areas that could have so-called "dual use" potential - that is possible military applications - may be declined visas.

MANUEL BALCE CENETA/AP US President Donald Trump withdrew from the Iran deal in August.

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One Iranian student said planned research work into improving water quality was scuppered by a sudden change of attitude from INZ. "It took [them] nine months to consider my renewal of my student visa, and finally they made these absurd allegations," the student said.

The research topic was suggested by his university and would have been conducted in partnership with a Crown Research Institute.

The number of Iranian students having visas declined by Immigration NZ almost doubled last year.

But Immigration NZ Manager Peter Elms said the number of declines was due to a rise in applications and most were declined for regular reasons, like a lack of funds.

Elms said INZ looked at factors including whether countries had a weapons programme, whether they were signatories to a non-proliferation treaty, if students were state sponsored, and if their study was relevant for "potential dual use". He said there was no particular focus on Iran and denied any outside influence on New Zealand policy.

"A lot of areas of study, on the surface, seem quite benign and civilian in use, and the vast majority of them actually are, but when we dig into the detail of a particular piece of research, it could have what we call dual use in a weapons programme," Elms said. "If you talk about water, it could be nuclear, could be chemical, even biological or for missile systems."

Elms said very few students were turned down on these grounds.

Iranian students spoken to by Stuff said it had become harder for them to secure visas with INZ taking longer to process them and demanding more paperwork. One said: "Absolutely, nobody wants to upset America on the world stage."

Another said the time taken to process their visa this year had "almost doubled" compared to last year.

One student said they had to fill in extra forms with "really offensive questions" like had they killed anyone in a war, another was warned off selecting a "controversial subject area".

SUPPLIED Dr Paul Buchanan says a US-driven clampdown by NZ would be contradictory to New Zealand policy.

International security expert Paul Buchanan said "there may be something" to claims from Iranian students that they were being placed under greater scrutiny in recent months.

"The evidence as to why a student needs to be prevented from continuing a particular course of study would have to be, in my mind, pretty darn compelling."

The Trump administration has recently signalled tighter sanctions on Iran.

In June, a newsletter for immigration advisors said INZ was working behind the scenes to assess "courses of study which may potentially contribute to weapons of mass destruction" and included an explanation of their work from Immigration official Nicola Hogg, who said the delays were from efforts to root out "would-be proliferators" from problem countries.

New Zealand Association for Migration and Investment (NZAMI) board chair June Ranson said one of the reasons she asked INZ to write the explainer was because long delays in visa processing had become "a pain".

RICHARD DREW/AP Israeli Prime Minister Benjamin Netanyahu speaking about the dangers of Iran acquiring a nuclear weapon.

Iranian-born lecturer AUT lecturer Dr. Pedram Pirinia, who was based in Canada before he moved to New Zealand, said extra restrictions for Iranian students were justified but was skeptical Iran would send students to New Zealand to advance its WMD programme.

"It makes me laugh," Pirinia said. "Believe me, if Iranian officials are looking to find the science behind nuclear weapons they will not send them to Auckland University."

Lawyer Alastair McClymont, who was acting for the water study student, said INZ's reasons for declining or delaying his student, and later his skilled work visa application, seemed to change "every time you seem to ask them what the reason is".

McClymont's was first told weapons of mass destruction we behind the delays for his client, then the country's association with human rights abuses and finally he was told countries that had not signed the 1968 Treaty of Non-Proliferation (NPT) were being targeted.

Iran was one of the first signatories to the NPT while India, Pakistan, Israel, South Sudan and North Korea were the only countries that hadn't signed the agreement.

The student said he believed "there are bigger forces at work here". His visa was approved two years ago but the world had changed, he said. "My feeling is that the New Zealand Government is being pressurised by another country."

Buchanan also said a clampdown on immigration applications because of US actions on the Iranian nuclear deal would be "contradictory" because NZ had opposed the move.

Elms also denied the student's claim. "Have the risks increased? Well, not really - it's been a very real risk in the international community for a lot of those 12 years. At any time in those past 12 years, there have been a range of countries actively seeking knowledge to develop their weapons programmes. The mix of countries changes.

"We don't take our responsibilities lightly, and certainly in the very rare occasions when we do decline visas, it is done for the right reasons, to protect New Zealand."