There are accusations the Corrections Minister is out of his depth, after more bad headlines for the Serco-run Mt Eden prison.

The latest performance tables show the prison has plummeted from the top to the bottom.

It comes at the same time as revelations youth offenders are kept in their cells for 23 hours at a time.

Green Party corrections spokesperson David Clendon said it's becoming par for the course.

"It's unsafe, some of the conditions which inmates have been kept are inhumane, there's really failing across the board. The only surprising thing is that is hasn't been revealed sooner." 

Clendon said Sam Lotu-Iiga always seems to be on the back foot, and surprised by these types of developments.

"That is a worry when he is ultimately responsible for the safety and security of 9000 inmates and many thousands of front line corrections staff."