Spectators saw the shark leaping in and out of the water near Coffs Harbour, close to the surfers and swimmers.
The moment was caught on camera by local resident Steph Bellamy, who said the shark “photobombed” her picture.
Surfers evacuated the sea for 10 minutes before resuming the contest, she said, adding that no one was hurt.
The encounter took place at the annual Coffs Harbour Boardriders Club competition at Macauley’s Beach, 340 miles north of Sydney, on November 30.
Steph Bellamy said she was taking a photo of the competition when she saw a splash.
She added that it was unusual for a shark to swim so close to the coast.
She showed the picture to the announcer on the viewing deck, who “got everyone out of the water”.
The photo has been examined by officials at the New South Wales Department of Primary Industries (DPI).
Spinner sharks are generally found north of Jervis Bay, and can reach a maximum length of 9ft.
Steph Bellamy said her photo had been widely shared since November 30, and the response had been generally positive, although some people had accused her of digitally altering the image.
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