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, despite an extensive search involving police, Defence Force personnel and large numbers of volunteers.
"Whilst we've all been hoping for a miracle, that miracle has not eventuated," SA Police Assistant Commissioner Ian Parrott told reporters on Friday.
"This is clearly now what appears to be a very tragic set of circumstances."
Dozens of searchers, including mounted police and divers, scoured the area and waterways surrounding the homestead during the week, covering nearly 500sq km.
No significant evidence was found to allow emergency services to focus the direction of the search, with police earlier admitting the footprint could have been a week old.
"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," Mr Parrott said.
Police would continue investigating the boy's disappearance, including the possibility he might not have got lost while playing outside the property, near Yunta about 300km north of Adelaide.
"But 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 and we've not been able to locate him," Mr Parrott said.
The senior officer had a pointed message for "keyboard detectives" discussing the case online, saying it was unhelpful and inappropriate.
"There's a lot of speculation on social media and various theories that quite a number of people are espousing," Mr Parrott said.
"If you were to put yourselves in the shoes of a family who will be clearly distraught about the loss of a small child, someone who's gone missing, it paints a little bit of a different picture."
Senior police 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.
"Life's not supposed to work where a parent loses their child, it's supposed to work the other way around, so I think it becomes incredibly traumatic and upsetting for families who lose children," he said.
The boy's family spoke earlier in the week, saying they were "struggling to comprehend what has happened" to the youngster.
"Gus's absence is felt in all of us and we miss him more than words can express," family friend Bill Harbison said on behalf of the family on Tuesday.
"Our hearts are aching and we are holding on to hope that he will be found and returned safely."
The "adventurous boy" had been playing in the sand near the family homestead when he disappeared.
