The shark was spotted 500 metres from the coast of Aldinga Beach about 8.30am yesterday.
Witnesses reported they believed the shark to be about 4-metres in length, having compared the animal to the size of their boat.
Surf Life Saving Patrols had not started for the day, so police were called to help alert the public.
The sighting has come after a fisherman reeled in a 2.5 kilometre shark at a Perth beach on Wednesday night.
CSIRO, Australia's leading scientific research body, has identified two great white populations in Australia; an eastern population and a southern-western population.
The eastern population is sized around 5460, but there could be more than 12,802 great white cruising the waters.
A number for the south-western population is "not yet determined".