"If the safety of staff and prisoners was a real priority Serco should've been relieved of their multi-million dollar contract much sooner."
He criticised Correction Minister Peseta Sam Lotu-Iiga's handling of the "shambolic situation."
The Minister said safety of staff and prisoners "in our facilities is paramount. They, and the public, need to be assured that a thorough review is being carried out.
"It is important the chief inspector has the time he needs to go through a large amount of information."
The final report on the first phase of the investigation will now be finished by October 30.
It includes the circumstances of prisoner-on-prisoner fighting, access to cellphones and the adequacy of reporting incidents of prisoner violence.
The terms of reference will now include the general safety and welfare of prisoners and reviewing monitoring arrangements.
The extension lets the chief inspector review complaints from prisoners and their families referred to him since the investigation was announced on July 19.
"It was necessary to extend the timeframes of the investigation due to the large amount of information being received and the need to carry out the investigation carefully," Corrections chief executive Ray Smith said.
"It is essential that the investigation is comprehensive given the level of public interest in the operation of Mt Eden Correctional Facility. The safety of staff and prisoners is our top priority."
The department continues to manage the prison as the investigation is completed.
Phase two will now be done by November 30. It reviews the adequacy of controls of prisoner violence and access to cellphones in other New Zealand prisons.
Corrections took over the running of Mt Eden in July after Serco came under fire following a series of serious incidents at the prison.
Videos have emerged of prisoners fighting and using drugs in their cells, and allegations have been made in Parliament about mistreatment of prisoners.
Serco was yet to respond to a request for comment.
