The Independent Police Conduct Authority is rejecting criticism contained in a United Nations report that it is failing in its role to adequately investigate complaints in custody.
The report by the Committee Against Torture looked at how New Zealand was implementing UN initiatives against inhuman treatment.
Its criticism is directed at the IPCA's role of monitoring police and court cells, for which it has responsibility under international law.
The report suggests the authority should have the right to lay charges against police found to have assaulted people in custody.
But IPCA chair Judge David Carruthers said overseas experience had shown shown why that did not work.
"Our relationship with the police is a very co-operative one. We get enormous Cupertino in all the aspects of work that we do.
"It doesn't mean that we are too close to them. It means that we stand back and if there are criticisms to make we make them public," he said.
"My experience is that other agencies which have a similar function overseas, which do have prosecution ability, end up without that co-operation and all that happens is that when anyone is under investigation they lawyer up, claim the right to silence and there's no helpful advance at all."
The Corrections Minister also rejected criticisms of the New Zealand prison system made in the report.
Peseta Sam Lotu-Iiga said New Zealand had one of the best corrections systems in the world.
"I don't know whether I agree with the assertions that they make based on the evidence that I've seen in our prison system."
Mr Lotu-Iiga said he would consider the recommendations, but he was comfortable with the state of New Zealand prisons.
In New Zealand, the power to lay charges rested with the police.
Judge Carruthers said because the IPCA had limited funds of about 55, 000 to meet its UN obligations but had had been working with police and would shortly release a set of national standards for cells.
"Because we have a limited amount of money we are trying to do this a different way.
"We're establishing, just about finished, this national standards for cells across the country, against which we expect the police to audit themselves. We'll also carry out an audit function as well."
This is the sixth such report on New Zealand's implementation of the Convention against Torture, Other Cruel, Inhuman or Degrading Treatment or Punishment.
The report identified 13 areas of concern including domestic violence, over-representation of Maori in prisons, the use of tasers and people-trafficking.
Among other matters it raised was the rate of violence between prisoners and assaults on guards at the Serco-managed Mt Eden prison which it said was higher than in public prisons. It said the Government should ensure private prisons are upholding the same standards as those in the public system.