The Police Association wants a damning report into police practices of taking photos and fingerprints to be thrown out, saying its recommendations will increase crime rates.
The joint inquiry, by police and privacy watchdogs, found police are routinely and illegally photographing young people and adults.
And police are also being accused of racial bias.
Police have been breaking privacy laws by taking thousands of photos with no clear and lawful purpose.
"This is a systemic problem. What we have is a situation where there are a lack of policies, practices and procedures," deputy Privacy Commissioner Liz MacPherson said.
An inquiry into the practice was launched after revelations in 2020 that rangatahi were being photographed by Wairarapa police.
"What we found gave us huge cause for concern," MacPherson said.
The inquiry looked at the way police collect data - including photos.
It found that of almost 11,000 people photographed between 2018 and 2021 - 53 percent were Māori.
"It's concerning because it raises issues of systemic bias within the police system, and within how police act as well," Māori Law Society co-president Baden Vertongen said.
The report recommends an overhaul of police privacy practices including a strategy for Police to better understand the Privacy Act, a system to store and delete photographs and for police to stop taking 'voluntary' fingerprints from youth.
"The implications of this is that we will struggle to prevent some crime and investigate some crime," Police Commissioner Andrew Coster said.
Police have accepted the findings about taking photos of rangatahi, but both the Commissioner and the Police Association said the wider recommendations will make police work difficult.
"We totally reject it. It goes far too far. It goes well beyond what the original complaints were. It creates a situation that's unworkable for police," president of the NZ Police Association Chris Cahill said.
"I don't believe that is the case. The privacy act is very flexible," MacPherson said.
Cahill said in particular police need to be able to keep copies of young people's fingerprints to solve crimes.
"We're aware of hundreds of positive fingerprint hits that would identify offenders of crime that police aren't acting on because of this report," he said.
He said if the report is not withdrawn by its authors the Police Commissioner should ignore it.
