The likelihood of finding a little boy lost in the outback is fading as family and searchers cling to hopes he has found shelter and is waiting to be rescued.

Defence personnel have joined the search for the four-year-old who has not been seen for almost a week.

August, known as Gus, went missing from his family's sheep station in the remote South Australian mid-north on Saturday afternoon.

The only trace found of the preschooler is a tiny footprint in the dirt about 500 metres from the family homestead, which brought hope, but police now admit it "could have been there a week".

"A four-year-old lad, they just don't disappear into thin air," Superintendent Mark Syrus said on Thursday.

"They have to be somewhere, so our job is to try and find which way he has gone and once we find thos

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