A young boy with golden curly hair smiles as he sits by a table with playing sand

Gus Lamont was reported missing on September 27. (Supplied: SA Police)

The disappearance of missing boy Gus Lamont has been declared a major crime by South Australian police.

At an update on the case, Major Crime officer in charge Detective Superintendent Darren Fielke said police had explored three “investigation options”.

They are that Gus had walked off from the station, that he was abducted or that someone known to him was involved in his disappearance and suspected death.

“The foot search has been unprecedented and the resources that have been used have been significant,” he said.

Gus was reported missing from his family’s remote property on Saturday, September 27.

A row of SES volunteers in orange and police officers in navy uniforms walk across arid ground

SES and police in the early days of the search. (ABC News: Daniel Taylor)

Police have said the four-year-old was last seen playing on a mound of dirt outside his family homestead on the sheep grazing station, about 43 kilometres south of Yunta.

The station is located about 350km north of Adelaide and spans about 6,000 hectares.

Gus’s disappearance just over 18 weeks ago has so far baffled authorities, shocked the nation and generated international headlines.

A search in outback terrain.

A police helicopter in outback SA during an earlier phase of the search for Gus. (ABC News: Daniel Taylor)

In the months that have elapsed since Gus went missing, multiple search efforts have been conducted, focusing on different areas of possible interest.