The opposition's crying corruption over the latest problems to beset Mount Eden prison, while the Minister of Corrections is waiting to see all the evidence.

Three inmates at the formerly SERCO run prison have been arrested as part of police drug operation against the Headhunters gang, a former prison guard is also being spoken to by police.

The inquiry's looking into the gang's role in the supply of banned goods within Mount Eden prison.

Labour MP Kelvin Davis said on the face of it, it looks like corruption.

"The only way that contraband could have got into the prison is via the guards."

Green Party corrections spokesperson David Clendon said the continuing problems are a major worry.

"I think it's sheer incompetence frankly, there is certainly evidence of corruption among individuals, but I think it's sheer incompetence."

New Zealand First MP Mahesh Bindra agrees, saying he knows for a fact the quality of staff that SERCO hires is far lower than the quality of staff hired by the Corrections Department.

"Serco don't pay the same wages, they don't have the same benefits, they don't have the same job security, and they don't have the same safety standards. So the quality of staff is definitely lower."

But Corrections Minister Sam Lotu-Iinga can't pre-empt the findings of the police investigation.

"It's not for me to comment on that. When the police finalise their investigation we'll go from there."

Mr Clendon said stronger action is clearly needed.

"The Minister has taken it seriously at one level, but he's taken it at a political level. He's failing, I believe, to make sure that the prison is safe."