"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.