Prepare to see more sharks this summer - scientists

New Zealanders face an influx of shark sightings this summer, as the warmer sea temperatures bring sharks and swimmers together.

"Both valuable and unwelcome marine organisms will increase in NZ waters with warming," NIWA scientist Cliff Law told Newshub.

"There's a global shift happening, with marine species moving either poleward or into deeper water to maintain their temperatures.

"As for sharks, warm water species may increase whereas cold water species decline."

Great white sharks have already been spotted in New Zealand's waters. Kiwi man Troy Kendall told Newshub he was surfing near Invercargill when he spotted a great white.

"About one-and-a-half-metres in front of me a fin and tail popped up out of the water, it was about two-and-a-half to three metres from fin to tail," he says.

"My first instinct was turn around and paddle as fast as I could back to shore. I got out of the water and got a better look and could easily recognise the length was huge."

The last fatal shark attack in New Zealand was in 2013 at Auckland's Muriwai Beach, when a man was attacked by a shark witnesses said was about 4m long.

Department of Conservation shark expert Clinton Duffy says shark sightings are likely to increase, due to La Nina conditions.

The water temperature in the Tasman Sea is well above normal - a whopping 6degC more than average for the start of December.

"There may be more sightings of rare tropical or subtropical species," he says.

"We may see more sightings of whale sharks, oceanic whitetip sharks, devil rays, and possibly manta rays around the coast of New Zealand.

"We may also see - depending if we see north-easterly conditions - we may see more things like the big game fish, like mako sharks, tuna and marlin."

La Nina climate systems mean there are less storms, which churn up the ocean and reduce temperatures. This will increase the chances Kiwis will spot a shark off our coast.

"If we get a prolonged period of calm water we are likely to see an increase in shark sightings," Mr Duffy says.

"That's mainly due to people being in the water and conditions being conducive to people seeing sharks."

The warmer seas may also lead to more nasties in our water. Bluebottle jellyfish normally start arriving later in summer, but the warm ocean means they're starting to be spotted now.

"Jellyfish blooms have been observed more often recently, and warmer weather may extend their reproductive windows, but hard to say whether they will increase," Prof Law says.

"Examples of other undesirables include viruses, infectious diseases and toxic algal blooms."