New Corrections figures show the controversial Mt Eden prison doesn't have the worst assault rate among inmates - but the union representing prison guards at state-run jails has cast doubt on the numbers.
The figures, released to Newstalk ZB, show the worst overall assault rate in the year to June was at Christchurch Women's prison, with 37 assaults per 100 prisoners.
Hawke's Bay Regional prison was next with a rate of 23.5 assaults.
Mt Eden prison, which has come under the spotlight following revelations of fight clubs and violent attacks on inmates, came in third with a rate of 22.9.
Mt Eden is run by private company Serco, although it is currently under management from Corrections while investigations into the prison take place. It recorded 224 assaults by prisoners on each other, or on guards.
The assaults in all prisons range from serious assaults that required hospital treatment to sexual assaults, spitting and pushing.
Beven Hanlon, of the Corrections Association, dismissed the figures as unreliable.
He accused Serco of under reporting serious assaults or not reporting them at all. Fox example, an inmate whose arms was broken so badly he needed surgery went unreported.
"If Serco are hiding the incidents that most normal people would deem severe and serious, you can bet your bottom dollar that they are also doing the same with the very small minor ones," he told Newstalk ZB.
Mr Hanlon said women's prisons often record high levels of assaults, but they were minor.
Better reporting also contributed to the increased numbers.
"They don't tolerate it essentially, so they report it. Remembering because of their very low numbers, one or two incidents pretty much put their stats through the roof."
But there were also inconsistencies in the recording of assaults.
"If a prisoner presents with a black eye and says that he slipped over in the shower, some people are motivated that down as an accident as opposed to an assault," Mr Hanlon said.
Corrections northern regional commissioner Jeanette Burns said: "The review that the chief executive has commissioned will look at Serco's management of [Mt Eden] and includes reviewing their incident reporting systems to ensure they are robust."
Corrections Minister Peseta Sam Lotu-Iiga declined to comment.
