An Aboriginal man who died in custody in 2015 repeatedly screamed for help and said he could not breathe before he became unresponsive and could not be revived, an inquest has heard.

David Dungay jnr, 26, died in the prison hospital at Sydney's Long Bay jail on the afternoon of December 29, 2015 after he was removed from his cell by a specialist riot response unit because he refused to stop eating biscuits.

Six officers from the Immediate Action Team forcibly moved Mr Dungay from Cell 71, which did not have video cameras, to Cell 77, which was under video surveillance, just after 2.42pm.

In an eight-minute interaction recorded by a handheld video camera, officers can be seen running into Mr Dungay's cell after a call of "go go go" and pinning him down to the bed and then the floor.