A new law that would allow prisons to punish inmates for drinking too much water has been criticised as "absolutely ludicrous".
The Corrections Amendment Bill aims to reform prison management systems, and includes a provision to stop prisoners from deliberately watering down urine samples for drug tests.
Under the proposed change, it would be an offence for a prisoner to consume "any substance with intent to dilute or contaminate the sample".
During the bill's first reading in Parliament last month, Corrections Minister Anne Tolley said some inmates would "water-load" to dilute their urine samples to try to avoid detection of drug use, and the law change was a further step in preventing that.
Wellington-based drug and alcohol counsellor Roger Brooking described the change as "absolutely ludicrous".
Mr Brooking, who is the clinical manager for Alcohol and Drug Assessment and Counselling, said that under the current rules prisoners who failed to provide a sample were automatically charged with misconduct.
For those inmates with "bashful bladders", the new law was particularly unfair because they might need to drink to be able to provide a sample, he said.
"Prisoners are now facing a kind of double jeopardy whereby they can be charged if they don't produce a sample, and if they drink water to help them produce a sample, which is not an unreasonable thing to do, they can get charged for that as well," he said.
"Prisoners are constantly struggling with incredibly petty rules, which enable the Corrections Department to charge them with misconduct and screw up their chances at the Parole Board, it happens all the time."
Labour justice spokesman Charles Chauvel said that rather than going through a court process, prisoners facing the charges would have their fate decided by appointed prison employees.
"Presumably the standards of justice that would be required internally are a bit less than a District Court," Mr Chauvel said.
While prisoners had rights of appeal to such decisions, the reality was that most prisoners would not go down those roads.
Mr Chauvel said he had heard "water-loading" was an issue overseas, but he was not aware of the extent of the problem in New Zealand.
"I'm not opposed to sensible measures that deal with drug abuse in prison, the question is whether this is one of them, and I think you'd have to say the jury would have to be out on it," he said.
"It's fair to ask whether this is the best use of Parliament's time, and whether this is a technique that's so widespread it's getting in the way of dealing with contraband in prisons."
The legislation also tightens up other rules around prison security, health systems, and gives more power to private prison managers by allowing the Corrections Department to delegate powers and functions to private contractors, including prisoner security classifications.
Labour, along with the Greens, the Maori Party and Mana, opposed the bill at its first reading.