The Corrections Association says the police have been investigating possible corruption at Rimutaka Prison for the past eight weeks.
In early August, police confirmed to RNZ there was an ongoing investigation into what it said were "matters" at the prison.
Police said the investigation, being run by the National Organised Crime Group had been running since July, prompted by information gathered in an earlier police operation.
Corrections Association president Alan Whitley said more than 70 officers had been involved in the investigation.
"All we've heard is that the investigation is into possible corruption," he said.
"We've been aware that the police have been doing an investigation there for quite some time.
"We don't know all the details of it, but it's been running for probably six to eight weeks now."
He said not everyone who had been interviewed as part of the investigation would be suspects, but could be witnesses or just part of gathering information.
Whitley said if there had been any kind of criminal activity they would expect to see charges soon.
The investigation was a good thing, he said.
"If there is something there, it needs to be found and rooted out; there is no tolerance for that in our line of work."
Police said the vast majority of the staff who had been interviewed had been co-operative.
"The investigation remains ongoing and no charges have been laid at this stage."
For operational reasons, police were unable to confirm the nature of the investigation.
The Department of Corrections said Rimutaka prison staff were assisting police with inquiries at the site.