Work to drain a dam on the South Australian outback property where missing boy Gus disappeared almost five weeks ago will be carried out on Friday, police have said.

The dam, which is about four-and-a-half metres deep, has already been inspected by divers, but police said draining it would allow a "comprehensive visual search" to be conducted, particularly in areas with submerged vegetation .

The development is the latest move by authorities endeavouring to solve the more than month-long mystery of the four-year-old's disappearance, which has rocked the state and been described by its premier as "one of the more heartbreaking cases I can recall".

Police have said Gus was last seen playing at his family's vast and isolated sheep station, about 40 kilometres south of Yunta, about 5pm o

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