If the average shark wasn’t already a terrifying force of nature, a helicopter patrol has reported seeing a seven-metre razor-toothed monster off the South Australian coast.

The great white was spotted about 100m offshore at Marino Rocks, about 20km south of the Adelaide CBD, on Sunday afternoon.

Its size was nearly comparable to the fabled Jaws which measured 25-foot (7.62m), however it may indicate there could be some truth to the mythical Hollywood creation.

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The Westpac helicopter crew compared the shark to a jet boat, which is about 6m in length, and the shark was bigger, according to the Shark Alerts South Australia Facebook.

One member of the experienced crew said it was a unique find.

“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 said on social media.

Nearby Nippers events were cancelled and swimmers evacuated from the water.

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”.

Turned off from the ocean forever?

A recent review carried out by a consumer advocacy group indicated in some circumstances that a shark repellent may help.

The devices, which come in many forms, shut down one of the senses sharks use to find prey, which include sight, hearing, a keen sense of smell, sensitivity to vibrations and electroreception, or an awareness of electromagnetic fields.

CHOICE looked into the effectiveness of electrical, magnetic, acoustic and spray repellants, as well as visual changes, like to the colour of surfboards and wetsuits.

It may seem far-fetched that the ferocious carnivores could be deterred by a small human device, but the investigation found although they “provide some peace of mind”, they had “limitations”.

They took particular issue with the fact that many devices had not been reliably tested by an independent body.

Electrical shark repellants are one controversial device available on the market.

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According to CHOICE, Shark Shield – which had been independently tested with success – could deter a shark, but would not do so every time. Meanwhile, anklet devices, like NoShark and Surf Safe, were found to be less reliable.

Magnetic and acoustic devices were also found to have limited success, while spray shark repellants, one of which contains a dead shark tissue extract, was “only an option if there is actually time to activate it, a luxury that may not be available in a real-life shark attack”.

“If you’re looking for a shark deterrent that’s 100 per cent effective, you’re going to be looking for a long time,” the CHOICE report stated.

“All the products we came across clearly state there’s no guarantee they’ll prevent a shark attack.

“One manufacturer even likened their product to a seat belt, saying that while seat belts reduce the risk of a fatality in an accident, they don’t completely eliminate it.”

– with AAP