The privately-run Mt Eden Prison has plummeted to the bottom of a performance table, as a report into allegations of assaults and fight clubs remains under wraps.
The Department of Corrections took over the management of Mt Eden Prison from Serco in July after the allegations surfaced.
In the latest quarterly performance table published by the department, Mt Eden was the lowest ranked prison - based on the 12 months to June - after previously topping the tables for nearly two years.
The department said the allegations of organised fights had been classified as a significant disorder event, which led to the performance downgrade.
Labour's corrections spokesman Kelvin Davis said Mt Eden's ranking had fallen in the blink of an eye, and he was concerned the government had not been getting all the facts until now.
Meanwhile, a draft report into the allegations has been completed but not released, after Serco went to court over it.