UPDATED 3.49pm: The Corrections Department has extended its investigation into Serco's management of Mt Eden Prison.

Chief executive Ray Smith reports the scope and time frame of the investigation has been pushed out because of the volume of issues raised.

The first phase of the investigation is focusing on prisoner-to-prisoner fighting, reporting systems and access to cell phones.

It will also look into the safety and welfare or prisoners, and monitoring arrangements.

The second phase will focus on controls on prisoner violence and access to mobile phones in other prisons.

Green MP David Clendon wants to know why a final warning still hasn't been issued to Serco given the new development.

He in concerned that although evidence is mounting that Serco has mismanaged Mount Eden Prison, no steps are being taken to get out the contract.

Corrections Minister Sam Lotu-Iiga has signalled it's not for him to say what the new matters are, and that they rest with the inquiry team.

"I think [the inquiry team] needs to investigate what comes forward. I shouldn't be getting in the way of the review that needs to be done."