Cleo Smith was last seen in her family's tent at about 1.30 am local time on Saturday (October 16) at the remote Blowholes Shacks campsite in Macleod, about 900 km (560 miles) north of Perth.
When her parents woke at 6.30 a.m. she was gone, police said. Local media reported that the family had been visiting for a weekend camping trip but were staying in the remote area.
Western Australian Police said they believe she was abducted from her tent and hold grave fears for her safety.
More than 100 police, defence force and state emergency service personnel have joined the searches by air, sea and land for the girl, scouring through the rugged area of the Western Australian coastline.