A mother and her young son are believed to be victims of a unit fire in south-east Queensland.

Their bodies were found in the burnt-out remains of a unit complex that caught fire on the Gold Coast overnight.

Forensic testing would be carried out to confirm their identities, police said.

Eight units were damaged in the fire on Back Street in Biggera Waters, which was alight when firefighters were called about 1.30am on Saturday.

Police have declared a crime scene as officers try to determine out what started the fire.

Chief Supt Mark Wheeler had earlier said emergency workers had had trouble gaining access to the site because of the extensive damage.