Police aren't yet able to say what caused a Townsville man to lose consciousness and die soon after he was taken into custody on.

The 39-year-old was taken into custody about 1.20am on Saturday after an altercation when he lost consciousness on a Mount Louisa street, Chief Superintendent Kevin Guteridge said.

"An ambulance was present at that time and despite extensive attempts by police and paramedics to revive the man prior to his transport to Townsville Hospital, unfortunately those attempts were unsuccessful," he said.

Police are unable to say if the man's death is the result of a pre-existing medical condition.

His death is under investigation by the Ethical Standards Command under the supervision of the coroner and Crime and Corruption Commission.