"For all deaths in custody, there is also an investigation by the independent Corrections Inspectorate.
"As the death remains subject to investigation, and the coroner is yet to determine the cause, we are limited in the amount of detail that we are able to provide."
Mikus was arrested in 2002 for the murder of Teresa in 1987.
Mikus had abducted, raped and murdered Teresa in 1987. He was finally arrested in 2002 using DNA evidence.
He was battling a brain tumour in Rimutaka Prison near Wellington and was being housed in a medical unit.
Former Napier detective Keith Price, now a long-serving Napier City Council, arrested Mikus and last saw the killer sitting in the dock as the officer gave evidence at the October 2002 trial.
He said he learnt of the murderer's death on Friday, and said: "I didn't grieve for too long - I didn't grieve at all."
Price said he had not seen Teresa's mother Kelly Pigott for some time and would know how she might react to news of Mikus' death.
"She has been very strong," he said.
"But it doesn't go away, it just doesn't go away."
Six-year-old Teresa's body was found face down in a shallow grave at Whirinaki Beach near Napier eight days after she disappeared on June 19, 1987.
Mikus got away with her murder for 15 years - but when advances in DNA enabled police to match him to samples taken at the time of Teresa's brutal killing, he was finally captured.
Teresa Cormack, 6, was murdered by Mikus in 1987.
He had been a suspect early in the investigation but had an alibi that was not challenged.
Mikus has been denied parole several times - most recently in April 2016.
At the time board panel convenor Justice Marion Frater said Mikus had consistently failed to engage with the parole process "from the beginning".
"Nor has he taken any steps to address his very serious sexual and violent offending. He has declined to participate in assessments for the Parole Board and declined to participate in any interventions with a departmental psychologist," she said.
"Given his assessed very high risk of sexual reoffending we are satisfied that even if Mr Mikus changes his mind and engages now, it will take at least five years before there is any prospect that he could safely be released.
"Accordingly, we make a postponement order of the maximum duration."
A spokesman for the Parole Board said his postponement order had not changed and no application had been made for an earlier hearing date or compassionate release.
After he was sentenced the Herald revealed Mikus had a raft of offences to his name, starting in his teens.
Brian Schaab, the detective who caught Mikus, died from cancer in 2017.
