It is not a welcome sight for school holidaymakers on the NSW north coast, but an "exciting phenomenon" of baitfish coming right to shore has brought 100 sharks in tow.

A huge school of fish swimming from Seal Rocks to Nine Mile Beach, Tuncurry, is being followed by a wide variety of marine life, and local commercial fishers say they have not seen anything like it in 15 years.

Great whites, bull sharks, whaler sharks, dolphins, Brydes whales and big numbers of sea birds have been stalking the group which is unusually close to shore.

However the reason they have come so far in is still a mystery at this stage.

NSW Department of Primary Industries shark biologist Vic Peddemors said he was very excited to see this marine event.

"This has been absolutely inspiring to get this new understanding," Dr Peddemors said.

"Just seeing this really big school of baitfish, the sharks, the dolphins, the whales feeding on it — it was just mind-blowing. Really fantastic."

One shark can be seen just metres in front of beachgoers. ( Supplied: NSW DPI )

This activity would normally be a lot further offshore, Dr Peddemors said, and scientists will work to understand what environmental factor brought the fish in.

"Maybe there's some extremely warm water in the area and these fish tend to be in cooler water, so maybe they are looking for cooler water," he said.

"To have them over such a large stretch of coastline suggests it has to be some kind of environmental event that has brought them so close."

Dr Peddemors compared the event to the famous sardine run in South Africa where billions of sardines migrate up the east coast followed by thousands of predators.

It is known as the greatest shoal on Earth.

"In my mind this is the New South Wales equivalent," Dr Peddemors said.

Be sensible

For those on the mid-north coast, swimming is not off the cards but caution is advised with so many predators about.



"To advise people to stop swimming is just unrealistic," Dr Peddemors said.

"Just be sensible — if you see a lot of baitfish in the water, or you see a lot of dolphins or birds diving in the water, don't go in.

"The chances are that there are other predators like sharks in the water."

It is important to scan the water before heading in and try to stay shallow, unless surfing, Dr Peddemors said.

The NSW DPI also has a Shark Smart app which shares information on shark sightings.