Double-murderer Jason Reihana was treated at Waikato Hospital earlier this month next to patients unaware they were sharing a ward with a convicted criminal.
Reihana, who was jailed for life in 2007, was spotted at the hospital in early February by an unnamed person who had been visiting another man on the ward, Fairfax has reported.
A spokeswoman for Waikato District Health Board told the Herald they were unable to confirm whether Reihana was in the hospital and said they couldn't comment on his treatment for privacy reasons.
Reihana was convicted of murdering his former partner, Teresa Gunn, the mother of his two sons, and her new boyfriend Andrew Grabner in a frenzied knife attack in Tauranga on December 11, 2005.
Armed with three knives, he drove from his Te Kawa home and killed them both in a frenzied attack.
A jury found him guilty on two counts of murder.
In 2007, he was jailed for at least 21 years - one of New Zealand's longest minimum-term sentences.
But the next month, Gunn's family learned Reihana was set to undergo a $1 million bone marrow transplant after being diagnosed with leukaemia.
The Department of Corrections said it had a "statutory olbigation" to ensure prisoners receive "the same standard of care that they would expect in the community".
"A prisoner's level of care, including room type, is determined by the relevant DHB and the assessment of the medical team. Health service providers working with a prisoner are provided with appropriate knowledge of the patient's background," Corrections said in a statement.
"As we also have a duty of care towards the prisoner to protect their privacy and security, we do not advise other patients of an offenders' identity."