The Corrections Department broke its own rules by giving high marks to private prison operator Serco shortly after a near-fatal attack on an inmate in Mt Eden jail.
Guards left Benjamin Lightbody lying in his cell with brain injuries while they filed paperwork and ate afternoon tea in mid-2013.
Corrections' inspectors failed the prison for security and safety, but the department then gave it a pass.
It has refused to explain why.
Corrections owes us an explanation on this, and its hard to see how there can be a good one. The least disturbing - that the people rating prisons weren't paying any attention to inspector's reports - paints them as complete muppets who shouldn't have a job. But its also easy to see how Ministerial pressure over a flagship policy could have played a role. And either way, it suggests that prison rating is something that shouldn't be done in-house, but by an independent, external body.