The post-mortem examination of a man who died in police custody on Sunday has been completed, however it will be two weeks until a toxicology report determines his cause of death.

The 20-year-old was taken to the Manukau Police station in the early hours of Sunday morning and was pronounced dead about 6.10am.

A post-mortem examination was undertaken yesterday afternoon but the cause of death was still unknown and would be be a matter for the coroner.

The man was previously known to police and would was due to be named tomorrow, Counties Manukau police spokeswoman Kimberley Mathews said.

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Counties Manukau police district commander Superintendent John Tims yesterday said the man's death followed a police callout to an address where his family were found trying to restrain him.

He was aggressive, violent and intoxicated on arrival and was arrested for a breach of the peace about 1am, Mr Tims said.

"The person was taken back to the police station where he remained violent and aggressive towards police staff.

"A doctor was called to see him, but because of his violent and aggressive nature the doctor could not see him.''

The man was found to be suffering from "laboured breathing'' about 5am. He was put into the recovery position and police staff performed CPR on him. Paramedics then arrived and took over, providing CPR for 40 minutes.

The death was a tragedy and police were supporting the man's family, Mr Tims said.

He said police custody officers were highly trained and knew how to keep people in that area safe.

"It's very early in the investigation process and we're just trying to gather as much information as we possibly can.''

Police have said the matter had been referred to the Independent Police Conduct Authority. There were also criminal and policy and procedures investigations under way.