An expert says a large shark filmed off a popular swimming beach near Nelson was very probably a great white - and it should come as no surprise to anyone.

Over the weekend, a large shark was spotted near Rabbit Island on Nelson's coastline.

Based on the creature's behaviour and shape in video footage, Department of Conservation marine scientist Clinton Duffy believes it was a great white shark.

Duffy told Morning Report sharks are performing their seasonal migration en masse from the tropics to New Zealand around now and could be found off most parts of the country.

Although our rarest large shark, great whites are seen regularly in the upper North Island throughout the year but as summer approached, they will become more abundant further south.

There sre ways people can keep safe if they encounter a shark in the water, he says.

"The most important thing to do if you see a great white shark is stay calm. If you're in the water, obviously you're going to get out of the water as quickly and as quietly as possible.

"If you're scuba diving, you should stay on the bottom, keep an eye on the animal and don't attempt to leave the water until it has moved away.

"Leaving the water, you're best to do that either (by) surfacing directly underneath the boat that you're from, or swimming along the bottom and getting out on the shore or on a nearby rock."

It's typical for white sharks to investigate any "novel object" they came across in their environment, by swimming up, looking at them and biting them "to see what they are", he says.

However, there are also many interactions with divers that did not result in attacks.

Duffy saysd people should treat any shark over 1.8 metres long as potentially dangerous, however, most sharks were "no trouble at all" and with summer approaching they expected to see more bronze whalers and hammerheads close to the shore.

DOC is asking for any sighting of great whites or other protected marine species to be reported to their hotline, 0800 DOCHOT (0800 362 468).

-RNZ.