The two Northern Territory officers were in the Coles supermarket in Alice Springs about 1.10pm on Tuesday when they were alerted to a confrontation between the 24-year-old and a security guard in a shopping aisle.
"The male behaved rather aggressively and was placed onto the ground by those police officers, he was later identified as losing consciousness," Assistant Commissioner Travis Wurst told reporters.
It was reported the young man was "placing items down the front of his clothing" and he had assaulted the security guard when confronted, he said.
After being restrained by the officers the man stopped breathing and was given CPR before being taken to Alice Springs Hospital where he was pronounced dead about 2.20pm.
A pathologist was flying to Alice Springs to perform an autopsy on Wednesday, with police treating the incident as a death in custody and preparing a report for the coroner.
When questioned by reporters on Tuesday, Mr Wurst was unable to say if the man had signalled he could not breathe, whether the officers were on top of him or how long he was restrained.
"The investigation is only in its preliminary stages, there's lots of work to be done by our investigative team to collect the evidence from those involved," he said.
"I pass on my condolences to the family of this person, it's a tragic set of circumstances.
"It will also have a significant impact on the security guards and the police officers."
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