Gus vanished from an outback sheep property after last being seen playing outside a homestead. (HANDOUT/SA POLICE)

Gus vanished from an outback sheep property after last being seen playing outside a homestead. (HANDOUT/SA POLICE) Credit: AAP

Police have promised to keep investigating the mystery disappearance of a preschooler from an outback property despite scaling back their search for the missing boy.

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 four-year-old was a tiny footprint in the dirt about 500 metres from the family homestead amid an extensive week-long ground and air search.

4yo missing in outback for over five days

“We will not rest until we can try and find the answer to why Gus has been missing, and hopefully, like I said before about the family, return him to them,” SA Police Assistant Commissioner Ian Parrott told reporters on Friday.

“We will now transition into investigation focus, (and) we will now continue to pursue ongoing lines of inquiry.”

Police are confident they did everything they could to find Gus.

“Everything that we have found to date, every piece of information and evidence that we have explored to date, indicates that … Gus has wandered off from this property,” Mr Parrott said.

“Despite our best efforts, we have not been able to locate him and unfortunately we are now having to scale back this search.”

Emergency service personnel would leave the property near Yunta, about 300km north of Adelaide, he said.

Family of missing boy release statement as search continues - video Dailymotion

“There will still be, obviously, a liaison with the family, as we continue to follow lines of inquiry and to do everything we can to make sure we’ve covered every single option in terms of what may have occurred at that property,” Mr Parrott said.

Senior police previously told the family Gus was unlikely to still be alive, based on expert advice about the outback weather conditions and clothing he was wearing when he disappeared.

“Whilst we’ve all been hoping for a miracle, that miracle has not eventuated,” Mr Parrott said.

“This is clearly now what appears to be a very tragic set of circumstances.”

Gus was described as an adventurous boy who had been playing in the sand near the family homestead when he disappeared.

No significant evidence was found to indicate where Gus may have gone despite the massive search involving police, Defence Force personnel and large numbers of volunteers.