Police say a mental health patient who died after being tasered by officers in Sydney's inner-west on Sunday afternoon violently resisted officers earlier in the day.

A critical investigation is underway into the death of the 30-year-old man at Camperdown.

Police have confirmed they arrested the man earlier that day after he was seen disorientated and running into traffic in Glebe.

The man was approached, he resisted and tried to run away, and a violent confrontation between him and officers continued, police say.

He was sedated and physically restrained and taken to the Royal Prince Alfred Hospital under the Mental Health Act, but later escaped.

Assistant Commissioner Mark Walton said police had to use capsicum spray and a taser, with up to six officers needed to arrest him the second time.

Again, he was non-compliant and aggressive, Assistant Commissioner Walton said.

"He's a large man, somewhere in the vicinity of 120 kilos and over 6 feet tall and he was resisting police," he said.

"In the circumstances, the police have apparently determined it was appropriate to use those instruments to attempt to detain him."

Police said the man lost consciousness and stopped breathing shortly after being handcuffed, and attempts to revive him were unsuccessful.

"The officers were acting to try and look after someone," Assistant Commissioner Walton said.

"Unfortunately it's not the outcome anyone wants.

"Unfortunately when people are non-compliant and resist we end up with contact."

A spokesperson for the Royal Prince Alfred Hospital said in a statement the man was initially cooperative with medical staff, but became agitated before fleeing the hospital.

"Despite verbal de-escalation attempts by security, nursing and medical staff, they were unable to prevent him from leaving the hospital," the spokesperson said.

"The patient was calm and cooperative for the majority of the time whilst in the emergency department."

"No staff were injured"

Police have not said how many times the man was tasered.

Police were not wearing body cameras, but in New South Wales the tasers issued are equipped with cameras.

A critical investigation team from the homicide squad will report on the circumstances surrounding the incident.

That investigation will then be subject to an independent review before being provided to the coroner.

NSW Health is also conducting an internal investigation.

Anyone in the vicinity at the time of the arrest is asked by police to contact Crime Stoppers.