Drug dealers, burglars and drink-drivers have walked free after a police botch-up led to officers carrying out illegal investigations.
In a major embarrassment for Police Minister Anne Tolley, officers returning to the force were incorrectly sworn in, meaning criminal charges had to be dropped.
It is the third time it has happened. Tolley described the previous errors as a "monumental cock-up" which forced Parliament to pass emergency legislation.
Labour police spokeswoman Jacinda Ardern said there were serious consequences. "Ultimately, people who should be accountable before the law won't be," she said.
Superintendent Richard Chambers said police were investigating how the error happened, just 10 months after a similar bungle. He was "very disappointed".
Police discovered on June 9 an officer in Wellington exercising constabulary powers without the correct authorisation.
An audit found a second officer in Counties Manukau in April was also exercising constabulary powers without correct authorisation. Both have since been resworn.
"As a result, a small number of criminal charges — at this stage approximately 10 — have been withdrawn by police," Chambers said.
The charges included possession of cannabis for supply, selling cannabis, dangerous driving, failure to stop, driving with excess breath-alcohol, burglary, minor theft and receiving stolen goods.
Tolley said it was "simply not good enough". "I'm advised it was human error, and there is no systemic issue, but the consequences could have been more serious. Police will need to decide what employment action to take. This is a basic but important task and police need to get it right every single time."
Last October, Tolley described a similar error in which 63 officers were unlawfully sworn in as a "monumental cock-up". Parliament passed a new law under urgency retrospectively validating the oaths of the 63 who had returned to the force in the previous four years.
Then-Commissioner Peter Marshall ordered an investigation and Labour politicians said it had to never happen again. The same thing had happened in November 2009.
Ardern said they were "silly mistakes that are having serious consequences". "A minister who has been so firm, you would have expected her to keep a close eye on how this was remedied to ensure it didn't happen again ... Three times is not acceptable and she really should have had closer oversight."
The mistakes have been made since changes were made to the Policing Act in 2008, when oaths sworn by returning officers had to be administered by the Commissioner of Police or a person authorised by him instead of by district commanders or inspectors.
They come weeks after the Herald on Sunday revealed that Counties Manukau police had altered crime figures to make 700 burglaries disappear.
• Read also: Editorial: Unease at more police sloppiness