John Key does not accept that gangs are running New Zealand's prisons but says they are "very difficult places" with some very rough individuals.
The Prime Minister was responding to revelations, first made public by ONE News, that inmates at Serco-run Mt Eden Prison had been holding organised fights behind bars and then posting footage on social media.
Mr Key told TVNZ's Breakfast that prisoners did not "run our prisons", but that they were difficult places.
"I accept there are gangs in prison and I think we have to acknowledge there are gang affiliations and some of these criminals are very rough individuals and some of these prisons are very difficult places," he said.
"But I'm actually confident, whether it's a public or private prison, that they are run by good people who do a good job."
He did not want to pre-judge anything until investigations had been concluded.
"I've been around a few prisons and I think for the most part our corrections system is run well. We don't know how widespread this behaviour is.
"If people are operating fight clubs or parties or whatever it might be, then that behaviour's got to change."
Videos revealed by ONE News included an apparent fight club, drug taking and musical performances.
Prisoners also appear to be accessing Facebook, despite a ban on internet use.
Prison operator Serco has launched an investigation, while Minister of Corrections Sam Lotu-Iiga says his department is also carrying out a wide-ranging inquiry into events at the maximum security prison.