Police are investigating the death of an Otago remand prisoner after the Independent Police Conduct Authority became involved.
Radio New Zealand has learned the police are examining how Jai Davis died in February 2011.
The IPCA received a complaint in March this year from a drug and alcohol counsellor, who believes medical care at Otago prison has grave problems.
Police say they have been investigating since Mr Davis died 2½ years ago but have had a full-time team on the case only since April because of new information.
Radio New Zealand News understands at least 70 people have been interviewed, including former prison officers in Australia.
The Otago coroner was due to hold an inquest into the death late last year but it was put off indefinitely.
Police have also reopened the files on another death, that of Richard Barriball, 42, in October, 2010, four months earlier in the same prison. A coroner found he committed suicide while on remand.
The complaint to the IPCA was made by Wellington drug and alcohol counsellor Roger Brooking, who has been working in prisons for 15 years. He said police had to be made to take the case seriously.
Mr Brooking said two prisoners died within four months at Otago prison and that showed it has a major problems.
There were 12 deaths in New Zealand prisons in the 2010-2011 year, including the two at Otago prison.
At Mr Barriball's inquest, the coroner criticised Otago prison's care, calling it sub-optimal because medical staff there had not given him strong painkillers he needed for an arm-injury, nor acted on reports he was in a vulnerable mental state.
The IPCA says it has directed police to review their file on Mr Barriball while it investigates Mr Davis's death.
Police say they expect to finish their investigation into the cases in the next few months, and will decide then whether to lay charges.
The IPCA says it is investigating their actions but is not giving a timeframe for its inquiry.
When asked about Jai Davis, Otago prison would not comment because of the investigations.
On the earlier death of Richard Barriball, the Corrections Department said there was no indication he posed a suicide risk.
In response to accusations that the medical care is not adequate, Otago prison said it got accreditation a year after the two deaths from the Royal New Zealand College of General Practitioners for the quality of its medical care.
Corrections Minister Anne Tolley says there have been no specific changes as a result of the deaths but Corrections has been improving its medical care. Ms Tolley says Corrections has been focusing also on lifting the quality of care in the mental health area.
She says prisoner deserve good health services but are entitled to no better services than anyone outside.
A prison reform group says medical care in jail needs much more scrutiny.
Prison Reform Society president Peter Williams QC says he has not heard of specific problems at Otago prison but in general medical care in prisons is far below that of the general community.
Mr Williams says its very unusual for the police to investigate a prisoner's death and there must be some reason for it because they would not be wasting their time.
Sociology Professor Greg Newbold of Canterbury University, a former inmate, says prison medical care is a lottery which depends on the staff and there needs to be much more scrutiny of it.
Definitely slower, says report author
The author of a Government report on prison health services says there is clear evidence prisoners are much slower to receive treatment than people outside.
Four years ago the Ministry of Health's National Health Committee commissioned the first New Zealand study of prisoners' health experiences.
Authors Michael Roguski says inmates' treatment was not of poor quality but was often too slow.
"Someone's done wrong; they've been sentenced and that is their punishment. What our participants were telling us was that they were being doubly punished so, yeah, they were in prison but non-timely receipt of health care, or non-receipt of health care, meant they were being continually punished."
Mr Roguski says little change seems to have been made since the report went the ministry.