Four-year-old Gus was last seen playing in sand on Saturday afternoon outside his family's homestead — a sprawling sheep grazing station in South Australia's mid-north.

Police said when his grandmother went to find him, he was nowhere to be seen, sparking a frantic search by the family who then called authorities for help.

Ever since, multiple agencies — including aerial, water, trail bike and mounted police, SES, community volunteers and sniffer dogs — have combed the arid area, with the only breakthrough so far a single footprint found about 500 metres from the station's homestead.

Police said about 100 people have been involved in the search each day, with about 70 of those on the ground.

The vast station, which spans about 6,000 hectares and is scattered with blue bush and red dirt

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