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.

Detective Superintendent Darren Fielke noted that due to these inconsistencies, a person residing at the station has ceased to cooperate with authorities. However, he emphasized 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.

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 stated. 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.