A man who died in police custody in the NSW Southern Highlands over night had been Tasered and sprayed with capsicum spray after a "violent" confrontation with officers at a McDonald's restaurant, police say.

Kevin Norris, 38, had been arrested and taken to Bowral police station on Sunday night when he slumped to the ground, unconscious.

Police and paramedics tried to resuscitate the Mittagong man, but he died at the police station. 

Police have launched a critical incident investigation into Mr Norris' death.

Assistant Commissioner Gary Worboys, the commander of the Southern Region, said the man had been acting in a "troublesome" and "disruptive" manner at the fast-food restaurant on Bowral Road at Mittagong about 8.50pm on Sunday.

McDonald's staff called police, and when two officers from Bowral arrived they were involved in a physical confrontation with him, Assistant Commissioner Worboys said.

"Inside the fast-food restaurant it was a violent confrontation," Assistant Commissioner Worboys said.

"When police first arrived, they spoke to the individual, there was some physical confrontation in terms of the arrest. Police also used capsicum spray and eventually had to Taser the man to get control of him.

"Two police were injured and had to be taken to hospital for treatment as a result of trying to arrest this man."

The confrontation is believed to have been captured on CCTV camera at McDonald's.

Mr Norris' collapse at Bowral police station is also believed to have been captured on camera, and will be examined by investigators.

Assistant Commissioner Worboys said the man, who was not previously known to police, was conscious and in handcuffs when he was taken to Bowral police station.

"He was being subject to the custody process and then he slumped to the floor. Police provided CPR and were also assisted by ambulance officers at the station," he said.

Assistant Commissioner Worboys could not say how many times he was Tasered, or whether he was injured before he got to McDonald's.

He said police were initially called to a domestic incident in Bowral, and, while officers were on their way there, they received a further call to say the man was at the fast-food restaurant.

Assistant Commissioner Worboys said he could not comment on whether an appropriate amount of force was used during the arrest while the critical incident investigation was taking place.

"It is only early days yet and we look forward to that investigation and that report to the coroner," he said.

"It would have been a difficult situation inside that fast-food restaurant, and violence, and police doing their best to arrest that man.

"The critical investigation team will no doubt put all that together for us and that will be provided to myself and also the coroner in due course," he said.

He said police officers regularly had to make tough decisions about the best method for making an arrest.

"They have a number of tactical options, and, indeed, that choice is up to them depending on the circumstances, the environment and the person that they're confronted with," he said.

"This situation shows how difficult policing is, and, indeed, police officers make these choices every day."

The two police officers suffered head injuries and were taken to hospital for treatment, Assistant Commissioner Worboys said. They had since been released from hospital and were recovering at home.

An autopsy is expected to be carried out this week to determine the cause of Mr Norris' death.

Assistant Commissioner Worboys offered his condolences to the man's family and friends.

"It is indeed a tragic set of circumstances and our thoughts go out to his family and friends. Also, to the two police officers that find themselves injured and at home now recuperating," he said.

David Norris said his older brother had just started turning his life around, AAP reports.

"He had problems his whole life, mixing with the wrong crowd," an emotional Mr Norris, 32, told AAP.

"But a couple of years ago he started getting his life back on track and doing well for himself."

A critical incident investigation team from Cootamundra Local Area Command will now investigate all the circumstances surrounding the death. That investigation will be overseen by the Professional Standards Command.

Police are appealing for witnesses to contact Crime Stoppers on 1800 333 000.

By Megan Levy

This story originally appeared in the Sydney Morning Herald.