Cleo Smith: Police offer A$1 million reward amid fears the four-year-old girl was abducted

Australian authorities are offering a A$1 million reward for information on the whereabouts of a four-year-old girl they fear may have been abducted from a remote campsite.

Cleo Smith was last seen sleeping in her family's tent at the Quobba Blowholes coastal camp in Western Australia early on Saturday morning.

Her mother said she discovered the tent open later that morning and the girl missing, along with her sleeping bag.

Teams are searching by air and sea.

"Someone in our community knows what happened to Cleo," Western Australia Police commissioner Col Blanch told a news conference on Thursday.

"Someone has the knowledge that can help - and now there's a million reasons why you need to come forward," he said.

Col Blanch said police were "chasing down leads" based on information they had already received.

Investigators said they were concerned that Cleo may not have left the area on her own because the tent's zip was opened to a higher point than she would have been able to reach.

"We are hopeful that we will find Cleo alive but we hold grave fears for her safety," Det Supt Rod Wilde said.

"Given the information now we have gleaned from the scene, the fact the search has gone on for this period of time... that leads us to believe she was taken from the tent."