A prison inspector has rebuked jailers for advising inmates to relieve themselves on vehicle floors.
One inmate said he was told to urinate through a drain hole while he was handcuffed in a vehicle moving from Wellington to Auckland.
The Herald on Sunday has obtained emails showing Inspector David Morrison's concern about inmate Brendon Forrest's complaints.
Forrest, in jail for arson, threatening to kill and perjury, is a repeat complainer - but Morrison found his concerns deserved investigation.
"Staff have been advising prisoners on long-haul escorts to urinate on their cell floors," Morrison wrote on April 16.
"Not only is this a breach of the design specifications but such practice could have a reaching adverse impact on the reputational risk of the department if such practice was to reach the media," Morrison wrote.
An inmate monitoring system revealed Forrest left Rimutaka at 6.48am and arrived at Waikeria Prison in Waikato at 4.29pm. He was transported to Paremoremo the next day.
Prison reformer Peter Williams QC said he had received several complaints about prison transport and "long-haul transfers" over the years but couldn't recall inmates being forced to urinate on vehicle floors.
Corrections Minister Anne Tolley told the Herald on Sunday: "I'm advised there are regular toilet stops, but public safety must come before prisoner comfort.
"If staff are ever found to have acted improperly then I would expect Corrections to put things right."
Liam Ashley, 17, was murdered by a 25-year-old while travelling in a van seven years ago.
A report into his death recommended prisoner access to toilet facilities be reviewed.