Police have scaled back the search for missing four-year-old Gus Lamont and handed the investigation to their Missing Persons Unit after failing to find any trace of him.

August 'Gus' Lamont vanished last Saturday afternoon from his grandparents' remote homestead, about 40 kilometres south of Yunta, north-east of Adelaide.

Hours after a photo of Gus was released by the family, South Australian Assistant Police Commissioner Ian Parrott made the difficult call on Friday with the sad admission that it's unlikely Gus will be found alive.

Despite a massive search effort—one of the biggest in the state's history—police have no significant clues regarding Gus's whereabouts, apart from a footprint located 500m from the property.

'While we've all been hoping for a miracle, that miracle was not eventuated,' Commissioner Parrott said.

'And in the last 48 hours, despite the professional advice it being unlikely that Gus would have survived, we have maintained and in fact increased the effort to try and locate him and bring him back to his family.

'We're confident that we've done absolutely everything we can to locate Gus within the search area, but despite our best efforts, we have not been able to locate him, and unfortunately, we are now having to scale back this search for Gus.

'We won't stop, we will continue to investigate and follow any lines of inquiry that we currently have... but everything that we have found to date, every piece of information and evidence that we have explored to date, indicates that, as best as we know, Gus has wandered off from this property and we have not been able to locate him.'

The tough call was based on scientific and professional advice on how long a four-year-old could survive alone in remote terrain in freezing overnight temperatures in the clothing Gus was last seen wearing. 

Police still hold slim hope that they will find Gus alive and return him to his family.

'But clearly that is that potential is diminishing as days go by, we will now transition into an investigation focus,' Commissioner Parrott said. 

'Gus was seen outside at 5pm on Saturday playing on a mound of dirt by his grandmother. At 5.30pm she has gone outside to call him in, only to find him missing.

'According to a family member, he is a shy but adventurous child. While a good walker, he has never left the family property previously.

'On Tuesday night senior police spoke to Gus’ family and prepared them for the fact that Gus may not have survived due to the passage of time, his age and the nature of the terrain he is missing in.

'This assessment was based on medical evidence from a doctor who is recognised by the Australian rescue authorities as an expert opinion on time frame for survival.

'While those involved in the search have been hoping for a miracle, over the past 48 hours the search has shifted to a recovery operation.

'At this point, no trace of Gus has been located. No tangible pieces of evidence, such as footprints, a hat or clothing, have been located to identify any direction of travel to assist searchers. 

'The investigation will now be managed by the Missing Persons Investigation Section. This is where all long term cases such as this are managed and investigated and is standard practice. 

'We will now continue to pursue ongoing lines of inquiry, and we will not rest until we can try and find the answer to why Gus has gone missing, and hopefully, for the family, return him to them.'

ADF personnel will be stood down from the search on Friday afternoon as the number of police vehicles surrounding the property significantly reduced.

Hundreds of searchers have scoured the red dust and scrub, but the only trace of Gus is a single footprint discovered about 500 metres from the property's homestead. 

Gus was last seen wearing a grey broad-brimmed hat, a blue long-sleeved shirt with a Minion picture from Despicable Me on the front, light grey pants, and boots. 

Police believe that Gus wandered off and was not taken, as the only people who travel on the nearby road are station owners.

Yorke Mid North superintendent Mark Syrus previously said it was 'unusual' for Gus to wander as he 'normally stays within the confines of the area'. 

'But, who knows what goes through a four-year-old's mind?' he said.

'We understand he's a pretty quiet sort of lad but he is, as you know, a country lad and he's pretty adventurous as well.

'But him moving out of the area is a little bit unusual.'

One local told the Daily Mail little Gus may have fallen into a mine.

'I would be more worried about the unmarked wells and mines he may have fallen into,' he said. 'That's the talk [among locals].'

The state's north-east pastoral district is dotted with mines and wells from a bygone era well over 100 years ago - and many of the region's station owners are still stumbling across them to this day.

They were used as water sources for livestock and for those 'chasing the gold dream' back in the day, according to our source.

Many of these sinister holes are virtually invisible to an adult's naked eye - much less a meandering child.

Family friend Bill Harbison issued a statement on the family's behalf earlier in the week.

'This has come as a shock to our family and friends, and we are struggling to comprehend what has happened,' it read.

'Gus's absence is felt in all of us and we miss him more than words can express.

'Our hearts are aching, and we are holding onto hope that he will be found and returned to us safely.

'At this time, we kindly ask for privacy as we focus all our energy on supporting the search and working closely with the police.'