Prison officers allegedly used "ghosting" to alter records after an inmate's death, to state that they had checked on prisoners more often than they had.
Jai Davis died at the Otago Corrections Facility on Valentine's Day 2011. Remand prisoner Richard Barriball had committed suicide at the facility on October 9, 2010.
Both men were considered to be "at-risk" prisoners.
The police investigations into the deaths of Davis and Barriball are now under scrutiny by the Independent Police Conduct Authority.
Davis was believed to have smuggled drugs into the prison when he was admitted on February 11.
He had hidden them in his body and prison sources said he was not checked overnight as was required. Records were changed to make it appear that he was checked.
A prison nurse told the Herald on Sunday that she had asked for Davis to be taken to a hospital.
He was found dead on February 14, and the actual time of death is believed to be at the heart of the police investigation.
Notes left by the officers who supposedly checked on Davis state he was moving around, the nurse said. "I've seen a lot of dead bodies but I've never seen one walking."
The death followed the suicide of Barriball in late 2010. Last year, Coroner David Crerar was highly critical of prison staff who delivered "suboptimal" care to Barriball, who was recovering from an arm operation and on four types of pain relief.
He was denied Tramadol and was given the much weaker Panadol, allegedly because doctors can withhold medications that can be traded in the jail.
The IPCA confirmed it was looking into aspects of the police investigations into the deaths of Barriball and Davis.
Criminologist Greg Newbold said he had never heard of a prison death being dealt with by the IPCA.
Corrections Association president Beven Hanlon said there was little a warden could do if a prisoner was determined to commit suicide.