Cleo Smith: Former Madeleine McCann detective believes young Australian girl's disappearance could be an abduction

West Australian Premier Mark McGowan has announced a A$1 million reward to anyone who has information that can help authorities find Cleo Smith.

Western Australia Police say a clue regarding the tent's zipper is a dead giveaway that someone else was involved in her disappearance.

Cleo has not been seen since the early hours of Saturday morning. She had been camping with her parents and baby sister at Blowholes campground, a tourist hotspot about 70km from Carnarvon in Western Australia. 

Her mother, Ellie Smith, told local media on Tuesday her daughter had woken about 1:30am on Saturday asking for some water. Cleo had been sleeping in a separate area of the tent with her baby sister, Isla. 

When Smith woke a few hours later, she found the zipper to the family's tent was open and Cleo - including her sleeping bag - were no longer there.

An intensive search-and-rescue operation has been underway this week, with soldiers now traversing the rugged and remote terrain to help find the four-year-old. 

"Until we can put our hand on our heart and say Cleo is not in the search area, we will keep doing what we are doing," Insp Jon Munday said on Thursday.

Investigators have already determined the zip was too high for the 4-year-old to reach without help. The family's tent has been taken away for forensic examination. 

Twenty registered sex offenders living nearby are also under the microscope.

"Sex offenders have been identified as being around the Carnarvon area. [They] have been spoken to and are being dealt with by the investigative arm of this investigation," Insp Munday said.

Cleo's disappearance has now drawn the attention of former London Metropolitan Police detective Mike Neville, who worked on the disappearance of Madeleine McCann - a 3-year-old UK girl who vanished from a resort in Portugal on May 3, 2007.

"The potential [is] very likely to be an abduction… the police will continue to treat this [as though] she is still alive until there is definite evidence that she's not," Neville said.

Thirty years at Scotland Yard, the headquarters building of the Metropolitan Police, has taught him one thing - "parents don't give up hope".

Smith and her partner, Jake Gliddon, are staying close to where they last saw their daughter.

"They don't know what to do. They don't want to leave and that's completely understandable," Insp Munday said.

But time is now against them - it has now been more than five days and still, there's no sign of their little girl missing in her pink pyjamas.