The search for 12-year-old Hussein Al Mansoory, a boy with Down syndrome and autism, is ongoing for the third day in Auburn, Sydney.

Emergency services, including State Emergency Service, police, PolAir, and the riot squad, have been actively searching throughout the night.

Hussein was last seen on Saturday morning at 10:30am near Auburn Memorial Park and had a later sighting on Norval Street the same night.

Residents are prompted to inspect their sheds, garages, and backyards for Hussein, who was wearing a white T-shirt when he disappeared.

Search efforts are concentrated on construction sites, which may provide shelter from the weekend's hot weather.

Investigations have ruled out public transportation use by Hussein and there is no evidence suggesting foul play.

The search operation will intensify in the streets close to the last known location of Hussein on Monday.