Police believe teenager Cassie Olczak spent most of the past five days in the Royal National Park without proper food, water or shelter.

The 16-year-old, who had been missing from the Sutherland Shire since Sunday, was spotted by a passing motorist wandering along the Princes Highway in Waterfall, south of Sydney.

It's believed she stumbled out of the bush after spending almost five days in bushland described by rescue crews as rugged and difficult to access. It was not a long distance from where she was last seen at Waterfall station.

On Friday, her mother Connie was lost for words when asked about finding her daughter.