An early morning welfare check will be investigated after a man died in an ambulance as police and paramedics tried to restrain him.
A critical incident has been declared, with the homicide squad and the police watchdog to determine the chain of events leading to the 45-year-old's death in Sydney on Tuesday morning.
Police arrived at a Waterloo unit complex in the city's inner south about 2.15am following a request from paramedics to help with a possible mental health episode.
Despite willingly accompanying officers to the waiting ambulance, the man became resistant once inside, leading police to attempt to restrain him while paramedics tried to administer a sedative, police said.
During the police restraint, the man entered cardiac arrest and was pronounced dead in hospital.
NSW Police Assistant Commissioner Peter McKenna said he did not know whether the man was sedated before the cardiac arrest but police body-worn camera footage of the incident would form part of the investigation.
"He became aggressively resistant to the ambulance officers and the police and had to be restrained," Mr McKenna said.
No tools other than handcuffs were used in the restraint and the man was not armed.
"The ambulance called police and called for urgent assistance," Mr McKenna said.
"Police and ambulance were there to help this person, there's no doubt about that."
While not confirming the exact circumstances of the man's restraint, Mr McKenna said sedation is generally used for the protection of ambulance officers or the patient themselves.
"It's usually an aggressive nature, something where they're being non-compliant or violent," he said.
The four officers directly involved will be investigated by internal homicide and professional standards officers with oversight from the Law Enforcement Conduct Commission.