Cleo Smith, who was last seen at around 1.30 a.m. local time Saturday, was staying in a tent at the Blowholes campsite in Macleod, according to a statement from Western Australia Police Force published Monday.
"Grave concerns are held for Cleo's safety," the statement reads. "WA Police are exploring all possibilities and have a number of investigative resources to assist including detectives from the Homicide Squad as well as Forensics and local detectives."
Cleo was wearing a pink one-piece sleep suit with a blue and yellow pattern, police said, adding that she was last seen sleeping in a red and black sleeping bag.
She is around 110 centimeters (3 feet, 7 inches) tall with hazel eyes and honey-blonde hair, the statement added.
The rural settlement of Macleod sits between the sea and Lake Macleod, around 30 miles north of the town of Carnarvon.
The family are from Carnarvon and are familiar with the Macleod area as they often go camping there, according to CNN affiliate 9 News.
On Monday, officers were searching a number of shacks along the coastline, the police said.
Police have appealed for anyone who stayed at the campsite or had been nearby to contact them.
Cleo's mother, Ellie Smith, posted an appeal for information on Facebook.
"It's been over 24 hours since I last seen the sparkle in my little girls eyes! Please help me find her! If you hear or see anything at all please call the police!" she wrote.
"Last seen 1:30am and gone when woken up at 6am from our shared tent. Very very unusual for Cleo," she added.
Police Inspector Jon Munday said the authorities were "throwing everything" at the search, but did not provide many other details, according to 9 News.
"It's been reported a sleeping bag is missing. Other than that I really am not at liberty to divulge what else we believe may be missing," Munday said, adding that marine searches hadn't found any sign of the little girl.
"We're comfortable that they (the police) have done the best they can," he said.