The 68-year-old was prevented from joining the flight out of Darwin on May 30 after federal officers received reports of him being intoxicated, Northern Territory Police said in a statement.
He was taken into protective custody and driven to the Palmerston Watchhouse where the custody sergeant and nurse deemed it necessary to take him to Royal Darwin Hospital for assessment.
Upon arrival at the hospital federal officers noticed the man had lost consciousness, prompting medical staff to immediately commence CPR, with resuscitation efforts proving successful.
He was transferred to the Intensive Care Unit in a stable condition for ongoing treatment for what was a suspected medical event, NT Police said.
The man died in the ICU on Saturday.
"The cause of the man's death remains undetermined pending a post mortem," police said.
"The incident is being investigated as a death in custody as the man was in the custody of the AFP at the time of him first losing consciousness."
NT Police will prepare a report for the Coroner.
The death comes as rallies continue across the country to demand justice for Kumanjayi White, who died on May 27 soon after being forcibly restrained by plain-clothes NT police officers in a supermarket in Alice Springs.
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