The head of Corrections says he has known about claims of an organised fight club in Mt Eden prison but has never had the evidence to back them up.
Video footage has emerged on social media of inmates fighting and smoking cannabis at the prison, which is run by the private company, Serco.
Warning: the video below contains graphic images, which may be disturbing to some viewers.
The president of the Corrections Association, Beven Hanlon, said the activities were detailed in a confidential Corrections Department report last July.
However Corrections chief executive Ray Smith told Morning Report he only saw the report on Saturday.
"If it had required my attention I would hope someone would have brought it to my attention and escalated it to me," said Mr Smith.
"When I read the report it was quite sobering and I looked at the report and I looked at what I saw in the video footage and I had wished it has been escalated to me earlier."
The Corrections Association said the report on the fight club allegations was being viewed by top Corrections officials in July last year.
Mr Hanlon said he had raised concerns about the prison 18 months ago.
At the time, he said, he was told an investigation would be carried out but heard nothing further. He said he saw a confidential report late last week.
"I can tell you now that on July 9 2014, the chief executive, the national commissioner and the northern regional commissioner all received that report and it doesn't make nice reading.
"Particularly 12 months later when we're watching it on video, the report tells you everything you've seen on the videos that have been leaked onto YouTube.
"We've got Corrections knowing that this stuff is happening and doing, clearly, nothing about it," he said.
"You can't tell me Serco don't know Corrections did this investigation."
Corrections Minister Sam Lotu-Iiga, who took up the role in October last year, said he did not know of allegations about a fight club, drugs and cellphones.
Labour Party Corrections spokesperson Kelvin Davis said he should have been told.
"He should have been informed in the briefing to the incoming minister about this situation around the fight clubs and they should have done something about it."
Mr Davis said other video footage showing inmates freely smoking cannabis and drinking prison brew showed Serco was a private prison failure. He questioned who was running the facility - the company, or the prisoners.
"It certainly didn't look like they were worried that a prison guard was going to stumble in and find them. It was pretty much like a dorm party at university, these guys are just having a good time.
"You can't tell me that if they were being monitored appropriately that they would be able to sit around, drink, smoke, do all the things they're doing, fight.
"You know Serco has let these prisoners down."
The internal investigation by Corrections and Serco will also look into the use of cellphones, which were used to post the videos onto YouTube.
Mr Lotu-liga has promised a robust investigation of all the allegations with particular attention to the last three months, to determine whether the activities are widespread or limited to specific units.
"What I expect is for the facts to be found, the reasons behind how this came to be and some possible solutions around how we solve it," he said.
The minister said the Office of the Ombudsman would monitor and review the investigation and its investigator will independently report on the matter.
Mr Lotu-Iiga said Serco had been put on notice over the incidents at Mount Eden.
