The four-year-old is believed to have wandered off from her neighbour's backyard at around 2.30 pm on Wednesday, where she was last seen with two dogs.

An extensive search and rescue operation is now being conducted across seven kilometres of rural pastures and dense woodland near Stormlea on the Tasman Peninsula, in terrain and topography that is considered "difficult".

An ongoing investigation into Shayla's disappearance has ruled out anything suspicious.

Tasmania Police hold grave concerns that Shayla may be injured in the dense bush, hiding out and too scared to call out or make herself known to searchers.

But they, and her mother Bianca, remain hopeful that Shayla will be found alive, with autumn weather and temperatures over the last few days and nights working in their favour.

Inspector Gavin Hallett said medical advice indicated that the "healthy and happy" four year old could still be alive.

He said they would not stop looking until she was found safely.

"My grave concern is that she may be injured in the bush," Inspector Hallett said.

"She is four years old, she won't present to us, we actually have to find Shayla. We have to get down on our hands and knees, we have to search everywhere.

"A four-year-old could be disoriented, she could be confused, she could be scared. She won't present to us, and that is where our concerns are held."

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Inspector Gavin Hallett said the current search is revisiting previously searched areas on the basis that Shayla may be on the move.

"We've checked and we have rechecked. One scenario is that Shayla is in one location, and is still mobile, and so we will go and check those areas again," he said.

"The effort is not reducing. It is maintaind 24/7."

Tasmania Police have used helicopters and thermal sensing drones, cleared nearby dams, searched neighbouring properties and brought in sniffer dogs to find the little girl.

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On Day 3 of the search, people on horse-back are scouring a nearby pine forest and Victorian sniffer dogs and aircraft have joined the search.

Tasmania Police Assistant Commissioner Jonathan Higgins said people have been reaching out from across Tasmania, Australia, and the world, to assist in the search for Shayla.

He said the operation had spoken to other jurisdictions with experience in successfully locating missing children.

"Tasmania Police are doing everything we possibly can to bring little Shayla back safely to her mum, and we will continue to do that," Assistant Commissioner Higgins said.

"There are many aspects to missing people, and it is all happening, playing out here with searches on the ground, people behind the scenes doing intelligence work and looking at maps and providing extra assistance where they can."

He said in rural searches, as opposed to urban searches, the aim was to get as many people on the ground as possible to look for the missing person.

"The mood is very professional. Whether it is our sworn members, volunteers, people who have come to help on horseback. There is a positive mood," he said.

"There will be apprehension, we are now a couple of days in, but the people who are here are searching, they are here for the right reason - to find little Shayla."

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Inspector Hallett said on Thursday that concerns were growing for the welfare of Shayla.

He said the search area was based on the distance of how far a four-year-old could travel.

This included a search on foot of a 1.2 kilometre radius and an air search with a 2.5 kilometre radius, which has now extended out to 7 kilometres.

Shayla has long brown curly hair and was last seen around 2.30pm Wednesday wearing pink leggings, a cream top, and gumboots.

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