Police will struggle to weed out gang members trying to infiltrate the organisation, and may have to slash frontline services if the government doesn't increase their budget, Police Association president Greg O'Connor says.
At the association's annual conference in Wellington on Wednesday, Mr O'Connor said "dozens of people" with criminal backgrounds attempting to join the force had been snapped by background checks.
But with no new government funding, they may not be able to keep up.
"Under a funding freeze, every part of police is under pressure and that includes recruiting. Recruiting is being centralised to one or two pods and in the districts there won't be the scrutiny," he said.
Mr O'Connor warned that cuts would likely be made to frontline policing to afford to roll out new programmes demanded by the government.
But Police Minister Anne Tolley, who also addressed the conference and praised police for their work cutting crimes rates, warned they will have to do even better without any more money in the near future.
While there will be challenges, she says police need to look at ways to "work smarter and better" and stay focused on "delivering the very best service we can for taxpayers".
That includes meeting new public service targets, such as crime-reduction goals and fewer court appearances for young people.