The shark was seen 100 metres offshore at the Marino Rocks, a Shark Alerts South Australia Facebook post says.
Nearby Nippers events were cancelled and swimmers evacuated from the water.
"It's the biggest I've ever seen as a crew member in the Westpac chopper. We put it down as 7m - the photo really doesn't do it justice," an unnamed crew member wrote.
The pictures caused a stir on social media, with some swearing never to step foot in the ocean again, and others arguing the great white was unlikely to be that large and labelling the sighting a "fisherman's tale".
Shark Alerts South Australia said the crew used their six-metre jet boat to estimate the great white's size.
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