The 25-year-old Aboriginal man was allegedly arrested without incident after officers responded to reports a man damaged window louvres and assaulted a woman at a retirement village in Darwin on Tuesday night.
He was secured in the rear cage of a police vehicle and a short time later was observed by officers to have laboured breathing, police said.
Officers began CPR before paramedics arrived, taking the man to Royal Darwin Hospital, where he was declared dead about 10:10pm.
He is yet to be formally identified.
The death is being investigated by the NT Police's major crime section detectives with oversight from the Professional Standards Command.
Northern Territory Police Assistant Commissioner Peter Malley said officers were called to Tiwi Gardens Retirement Village over reports a man had assaulted a female resident.
They found the man lying in bushes.Â
"We took him into custody, handcuffed him and put him in the back of the police cage," Mr Malley told reporters on Wednesday.
"It was identified pretty quickly that his breathing was a bit laboured."
Officers were told an ambulance wasn't available and continued monitoring the man before he was removed from the cage and started CPR before an ambulance arrived and took him to hospital.
Mr Malley said there was no struggle when the man was put in the cage.
"I've actually watched the body-worn video of the whole incident and there's no resistance from the gentleman, there's no force used by police," he said.
"He's handcuffed to the front and he's lifted and placed into the cage. Very gentle."
A post-mortem examination will be carried out on Thursday to determine the cause of death and police will prepare a report for the coroner.
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