Police say a suspect has been identified over the disappearance of a four-year-old boy who went missing in the South Australian outback.

Gus Lamont was last seen playing outside his home on a remote sheep station near Yunta, about 300km (186 miles) from Adelaide, on 27 September.

His grandmother left him alone for about half an hour before checking on him, only to find the boy missing, prompting one of the largest land and air searches in the state's history.

Police said a person who lives on the property has been identified as a suspect but confirmed the boy's parents were not under investigation.

In the days and weeks after the boy was reported missing, police conducted extensive searches, covering about 470 sq km - an area roughly twice the size of Edinburgh - around his home, the Oak Park station homestead.

In late October, police wound down efforts and appointed a 12-member taskforce to continue the investigation.

That taskforce sifted through previous statements given by family members which "identified a number of inconsistencies and discrepancies" of the timeline around the boy's disappearance.

"As a result of these inconsistencies, and investigations into them, a person who resides at Oak Park station has withdrawn their support for the police and is no longer cooperating with us," Detective Superintendent Darren Fielke said.

"I do want to stress, however, that Gus' parents are not suspects in his disappearance."

Police conducted a search of the property in January and seized a number of items including a vehicle, motorcycle and some electronic devices.

Investigators said they had initially considered three possible scenarios - the boy walked off, he was abducted, or someone known to Gus was involved in his disappearance and suspected death.

Given the remote location of the property, police ruled out abduction and there was no evidence to suggest Gus had "merely wandered off".

"What I can say, however, is that we'll continue to thoroughly and meticulously investigate the disappearance of Gus until we get an outcome," Fielke said.

"We're all focused and determined to locate Gus and return him to his parents. Nothing is off the table as we work towards that outcome."