UPDATED 3.22pm: A man who's been incarcerated in the Serco-run Mount Eden Prison is backing claims of a regular fight club there.

LISTEN ABOVE: Former inmate 'Kenny' talks to Danny Watson about prison fight clubs.

"Kenny" told Danny Watson "contender" matches happen in just about every unit to sort the weak from the strong.

"I can recount 16 stitches in my head from my second contender match."

"When you get into these units you don't have a choice. You either fight back or you get the crap kicked out of you basically."

Kenny says the problem is a lack of staff - with around five guards watching over 120 prisoners.

The claims lend weight to those being made by Labour.

Labour's Corrections spokesperson Kelvin Davis is adamant a review of Mount Eden prison is not enough.

He claims a prisoner recently died from a ruptured lung, after being dropped from a balcony onto concrete.

Mr Davis said the prisoner, Mr Evans, was transferred out of Mount Eden to Northland's Ngawha Prison - before being sent to hospital where he died of his injuries.

He said it's just one example of a number of cases of unacceptable prison behaviour.

However, the operator of Mt Eden Prison says if the practice of 'dropping' was taking place, it'd expect related hospital admissions.

SERCO says it's never received any reports of the practice, and neither has Corrections.

Director of operations, Scott McNairn, said he would expect prisoners to be admitted to hospital on a regular basis with injuries consistent with being dropped, if the practice was taking place.

But he said there have been no such hospital admissions.

Corrections Minister Sam Lotu-Iiga has accused Mr Davis of making it up and says the prisoner died from a super-bug.

But Kelvin Davis said dropping is common in prisons and it comes down to poor management from SERCO.

"I'm not so naive as to say there's no violence in prisons. But you just need to look at the videos that have popped up on Youtube."

"The prisoners are in control of the prison - it's just a mess."

Lotu-Iiga has ordered a review of the prison, which is currently underway.

But Kelvin Davis said more needs to be done.

"It needs to be a full blown investigation. It's all these allegations. The terms of reference are pretty narrow, it's going to focus on the behaviour of these prisoners, rather than on the behaviour of Serco."

And Kelvin Davis claims there's been another case of another inmate being transferred out with two broken legs.

"I'd love to know, if the numbers of incident reports, and the number of medical reports, actually correlate."

Prime Minister John Key is challenging Labour's Kelvin Davis to come up with evidence to prove his claims.

John Key said it's not good enough for Mr Davis to make the claims and then simply walk away.

"Effectively these allegations are of murder, so if he's actually right, he has a moral obligation to go to the police."

He said the cause of Mr Evans' death is being investigated by the Coroner.

Prison operator Serco said neither it nor Corrections have received any reports of the alleged practice of dropping.