Prison guards are conducting thousands of strip searches but finding virtually no weapons or drugs.
The searches became more invasive three years ago, and at about the same time peaked at around 4500 a month, newly-released figures show.
But the hit rate on contraband in 2013 of 20 or so items a month is the same as now, even though strip search numbers have plunged by two thirds to 1500 a month across all prisons.
The Official Information Act (OIA) figures released to a lobby group angry at transgender inmates getting strip-searched by male guards, have shown up the ineffectiveness of strip searching - though the Corrections Department would not concede as much.
Paul Wood was strip searched thousands of times during the decade he spent locked up for murdering his drug dealer when he was 18.
"When you first start getting strip searched and you find yourself in a small room with two men who are fully dressed wearing all of their Corrections uniform and rubber gloves - it's a very off-putting and humiliating experience," said Dr Wood, who was now a leadership consultant.
He got out of jail in 2006.
Since then it has become tougher, in particular under a law change in 2013 that was opposed by the Ombudsman with warnings of going back to a "dark ages" of prison policy. The new law allowed guards to order male inmates to lift up their scrotums, and that was now routine in strip searches.
Corrections Association president Alan Whitley said that change ratcheted up the invasiveness.
"Strip searching is quite an offensive part of what you do and prisoners can react to it from time to time," said Mr Whitley.
"We would have preferred the status quo to remain 'cos we believe that as we were searching before, we were finding it - the fact they have changed the law and we have got to do it differently, we are still finding it."
But prison officers were not finding much of the concern.
The new OIA figures showed that in May this year, slightly more than 1600 strip searches discovered 21 smuggled items.
In March 2014, double the number of searches turned up 27 items; a year before that, just after the law change, 4217 searches unearthed 19 items.
And just before the law change - in September 2013 - about 4200 searches saw 27 items seized.
Statistician and Auckland University professor Thomas Lumley has crunched the figures and said they suggested strip searches could be cut back further with possibly no extra risk.
"If it's possible [for search numbers] to fall that far without a fall in the number of items found, that suggests that it could probably fall further, that maybe the prison officers are actually better than they think at identifying the highest risk people, and there's a lot of additional searches that are not actually finding anything."
For its part, the Corrections Department said it did not believe it was overusing strip searches.
"The power to strip search prisoners is one of Corrections officers most coercive powers and impacts on a prisoner's right to privacy and dignity. However, in appropriate circumstances it is a necessary imposition on those rights to ensure the safety and security of prisoners and staff."
In addition, last night Chief Custodial Officer Neil Beales said in a statement: "The searches are carried out with decency and sensitivity, and every effort is made to ensure that the person's privacy and dignity is maintained."
Guards' union president Alan Whitley backed that up.
The Department puts the paltry amount of contraband seized down to so many searches being routine ones, such as when a prisoner was moved between jails or to court.
It explained the drop by two thirds in the number of monthly searches - from 4500 to 1500 - to fewer court movements because of audio-visual links and fewer prisoners on temporary release for work.
Mr Whitley said the drop was odd, especially given the prison muster was at record levels.
Dr Wood said he suspected another factor - fewer visitors being allowed in.
He said strip searching was necessary, especially to intercept weapons, but it was not very effective and had a real downside.
"It really further reinforces the adversarial natures of the relationship between prisoner and prison guard. It does nothing to teach you about personal responsibility, the idea of not breaking the rules 'cos that's the right thing to do, rather than because you risk capture."