Reihana was jailed for at least 21 years - one of the country's longest minimum-term life sentences - for killing his former partner and her new boyfriend in a frenzied knife attack in a Tauranga home in 2005.
Tauranga woman Teressa Gunn and her new partner Andrew Grabner were killed at the home on Mansels Rd in Parkvale.
"The hairs stood up on my neck."
'If anything went down, what are you going to do?'
Reihana appeared to be suffering from kidney stones and did not show any signs of aggression during the time the two men shared a ward, the man said.
But he was still so rattled he turned down the option of staying an extra night in hospital, the man said.
He had no problem with Reihana receiving treatment, but believed the prisoner should have been in a private room.
The patient was particularly upset Reihana was left alone when the guard went for coffee or bathroom breaks.
"I felt quite vulnerable . . . some people in the ward were not very mobile. If anything went down, what are you going to do?"
Corrections, health board respond
Corrections' chief custodial officer Neil Beales said in a written statement that the department was unable to comment on the specific custodial or health management of a prisoner.
"We would like to reiterate that each prisoner's care plan is determined on a case-by-case basis, in consultation with the relevant health provider, and this is based on a range of factors, including their assessed security classification and known risks.
"This determines the level of supervision, restraint [such as handcuffs] required and required numbers of staff escorting the prisoner concerned."
Anyone with concerns about the supervision of a prisoner in a public area should contact the department as soon as possible, so their concerns could be investigated, Beales said.
Decisions about where patients are accommodated are made by hospital staff.
Waikato District Health Board spokeswoman Lydia Aydon said in a written statement that the DHB tried to place prisoners in single rooms, but this was not always possible.
"Prisoners are entitled to the same healthcare treatment and patient privacy as anyone in our hospital and we have a good process with Corrections to ensure the safety and security of patients and staff.
"Corrections officers always attend the hospital with prisoners . . . I'd like to reassure patients that their safety is always our priority."