The deaths in custody watchdog demands an independent investigator to be brought in to look at the death of a man in a Kalgoorlie watchhouse yesterday morning.

A 51-year-old man died in his cell after being arrested on Friday night.

Deaths in Custody Watch Committee chair Marianne Mackay said an outside investigation is needed, 6PR radio reported this morning.

"We are concerned that internal affairs who are conducting the investigation, because it's police investigating police," she said.

"From a deaths in custody point of view we prefer to see an independent investigation."