A leading researcher on gangs says he is being muzzled by the police, who have blocked him from obtaining basic information.
Jarrod Gilbert, a sociologist at the University of Canterbury, said police had told him he could not get simple data on where incidents such as assaults happen because of his association with gangs.
He said that association should come as no surprise.
"I entered the field as an academic and did the two largest studies on gangs this country's ever seen.
"So in effect I'm being banned from studying crime, because I hang out with criminals, it's just insane."
He wanted the data for research on alcohol and violence for another government agency and he said the decision not to provide it was outrageous.
Dr Gilbert is also critical of the contracts police ask researchers to sign when they do release data.
"I found the research contract which said that the police can veto any findings of a project they don't like and blacklist any researchers who disagree with that."
He said the language police use is Orwellian.
"What they're saying is, if you come up with something that they don't deem that they like, heaven forbid it's critical of the police for example, then they can veto it, they're controlling the academic process.
"Obviously that's got very chilling implications for free speech."
Dr Gilbert told Morning Report police initially gave him no explanation as to why he had been denied access.
"Subsequently, after pulling teeth to get some communication with them, all they told me is I've got associations with gangs, well again very few people will be surprised by that."
He said security clearance had never been a problem previously and he had been cleared by police for research on behalf of Corrections, to enter prisons in Canterbury, and for his participation as a volunteer firefighter.
"There's nothing in my file that concerns police, what concerns police is sometimes I'm critical of them, and they may not like the research that I engage in, that's not their business."
He said police were wielding "a very big stick" and that so far no academics had spoken out against the research contract which had been in exitstence for many years.
A standard clause says police retain the right to veto any findings from being published.
Police said they were reconsidering Dr Gilbert's access to data and deputy chief executive of strategy Mark Evans issued a statement.
"Police receives a large number of individual requests for academic research - 37 applications in 2014.
"While Police already publishes large volumes of data, such research requests often involve access to confidential information, which can include personal identifiers."
Police carried out vetting on individuals as part of ensuring the application was of good standard, met privacy obligations, and were feasible in terms of police time and resources, he said.
"The research agreement which academics are expected to sign with Police sets out our expectations, including that research is accurate, balanced and constructive ... Police also reserves the right to prevent further access to Police resources if a researcher commits any breach of the agreement.
The statement said that to date police had not prevented access by any academic under this clause in the agreement.
"While we will not discuss specific matters regarding Dr Gilbert, we can say that we have communicated to him that further consideration will be given to our decision regarding his police vetting check."
The union representing tertiary staff says the withholding of information by government departments was widespread and created a climate of silence.
It said Dr Gilbert's experience was not an isolated one.
National president Sandra Grey said government departments were wary of damaging headlines and that had led them to clamp down on public commentary by researchers.
Dr Grey said it was a breach of academic freedom.
"Over time it has got tighter and tighter, and a lot of this is about not wanting to be embarrassed publicly by research findings. But I think government departments should be bigger than that, and if it's in the public interest, it should be out there."
Dr Grey said academics had a legal duty to act as the critic and conscience of society.