El Nino is expected to bring out more sharks, and a scientist says there are steps we can take to ensure we do not encourage sharks to hang about near human activity in the water.

A teenager was killed in a shark attack on Thursday off South Australia's coast. It was the third fatal attack in South Australia this year.

Shark attacks have also already been reported in New Zealand waters this season, including by a diver on the Wairarapa coast and a woman who was in knee-deep water in a Southland estuary.

This summer El Nino is bringing a shift in temperature to New Zealand's coastal waters, which is expected to draw out more fish life - and more sharks.

Marine scientist Riley Elliott said El Nino contrasted to La Nina climate patterns in the way the wind blows across the Pacific Ocean.

"During La Nina, which was the past three summers, the wind is blowing predominantly from Chile to New Zealand. And what that does is push across the surface water all the way across the ocean to us. And in that time, that water has a long time to heat up."

El Nino, Dr Elliott said, blows the opposite way, dropping the water temperature about 5 degrees.

Dr Riley Elliott Photo: Supplied.

It also creates conditions conducive to increased fish life.

"It drags up water from our depths as it has the pull-up, like the conveyor belt, from New Zealand to Chile and that creates upwelling of nutrient-rich water that's cold but creates a lot of productivity."

And as a shark expert, Elliott was keenly watching the sharks.

During the beginning of summer there had not been nearly as many sightings as the same time last year, he said. But where there were more fish, the sharks would follow.

How to stay safe in the water with more sharks about

Elliot said more sharks would not lead to more shark attacks.

Sharks, especially those such as bronze whalers, do not pose a huge risk to people unless they overlap with the fish.

But he said it was important to remember sharks are predators, and there are things people can do to reduce risk.

"We've got to remember [the] ocean is a wild environment and it is one controlled for the health of it by sharks. And if we go play in it, you know - educate yourself on sharks, on what you should and shouldn't do."

He said people fishing should avoid throwing fish carcasses overboard. If they see sharks catching the fish on their line it was best to leave the area.

And swimmers should not swim where people were fishing.

However Elliott said drowning statistics significantly outweigh shark attacks.

In the lead-up to summer, Department of Conservation marine scientist Clinton Duffy also gave RNZ his advice for staying safe from sharks.

First and foremost, Duffy said, 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."

New Zealand has about 66 types of sharks, ranging from only 27 cm long, up to 12-metre long whale sharks, DOC says.

Dr Elliott tags great white sharks to track their movements, and encouraged anyone who sees one to get in touch. The Department of Conservation would also like to hear about shark sightings, captures or strandings, sharks@doc.govt.nz or via 0800 DOC HOT (0800 362 468).

-Krystel Gibbens/RNZ.