Police are refusing to reveal the possible cause of death of a man in their custody last night.

The man was found dead in the Rotorua police cell block at 8.45pm and an investigation into his death has been launched.

A post-mortem examination is expected to be carried out later today, and the man's name and other details have yet to be released.

Rotorua area commander Inspector Bruce Horne told the Herald that police had an idea about how the man had died but would not disclose it publicly.

"We do have an idea as to events but there's a lot of things we still don't know," he said.

Asked why police would not reveal the possible cause of death, Mr Horne said:

"It's obviously a serious matter. It's a matter that needs to be reported to the Independent Police Conduct Authority. There are protocols that need to be followed."

Earlier, he said the man had been arrested and charged with assault with intent to injure as a result of a family violence incident.

He said a police investigation had been launched and the matter had been reported to the Independent Police Conduct Authority.

Mr Horne told the Herald that police were called to a domestic incident involving the man at 5.15pm yesterday, but that determining the subsequent timing of events was part of the police investigation.

He would not disclose where the domestic incident had occurred, other than to say it was in Rotorua, and said police had no further comment on the matter at this stage.

"There's nothing else we can say. There will be an investigation to determine what occurred."

Mr Horne said police were still working on contacting the man's next of kin and no decision had been made on when his name and other details would be released.

"We've got to talk to the family first," he said.

The process of tracking the man's next of kin would continue when staff who had been on duty last night returned to work at midday.

Mr Horne said police needed time and space to conduct their investigation, and declined to answer further questions.

NZPA reported that an investigator from the Independent Police Conduct Authority had been sent to Rotorua to begin an inquiry into man's death.

A spokesman for the authority said its investigator, a former senior police officer, was visiting the scene this morning.

By Juliet Rowan | Email Juliet