Police have renewed search efforts for missing South Australia boy Gus Lamont as detectives prepare to drain a large dam on the family's property.

Gus, aged four, was last seen on September 27 at his family's homestead in Oak Park Station, about 40 kilometres south of the town of Yunta and 350 kilometres north-east of Adelaide.

Police have confirmed a fresh search on Friday will involve draining a 4.5-metre-deep dam on the family's property, situated about 600 metres from the homestead.

Detectives said the dam was being searched to rule out the possibly that the four-year-old drowned.

"The draining of the dam will enable a comprehensive visual search to be completed, particularly areas with underwater vegetation," police said in a statement on Thursday.

"The renewed search of the dam

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