A woman has suffered a serious shark bite while walking in knee-deep water in Southland.

The 21-year-old was bitten on the leg about 2am on Monday while in the estuary near the Riverton rugby grounds, a police spokesperson said.

"She received a significant laceration to her leg that required treatment from emergency services.

"The culprit is possibly a sevengill shark, which are present in the estuary. The time of day is a possible factor."

People needed to be cautious while in the water and conscious of their environment, police said.

"Riverton is a popular holiday destination, and police ask that anyone heading into the water follow safety guidelines and avoid late-night swims."

A St John spokesperson said it was notified at 2.27am.

An ambulance and a rapid response vehicle went to Leader Street.

"We treated a patient in a serious condition and then transported them to Southland Hospital," the spokesperson said.

According to NIWA, the sevengill shark "is a bit of a biter" and can become aggressive if provoked.

"This shark has serrated teeth, which are slanted sideways with multiple sharp cusps."

NIWA said they grew to three metes are are found all around New Zealand in shallow to deep waters, but were more common in the North Island.

The sharks are browny-grey in colour and have small black and white spots with a white belly, NIWA said.

'You just don't know what's out there'

Riverton Coastguard president Ross McKenzie said he was surprised to hear of a shark attack at the estuary.

"As far as I am aware that's the first shark attack of that kind in the estuary around Riverton. I have been here around 20 years and it's the first I've heard anything like that," McKenzie said.

However, local anglers saw and caught sharks quite regularly while fishing off the wharfs in Riverton, he said.

"They're there all the time and the local fishermen and guys that go fishing off the wharfs do see them regularly especially at night.

"You would reasonably assume you'd be safe wading around in knee-deep water, but you just don't know what's out there and it is their environment, not ours."

Plenty of people flocked to the small seaside town over the holiday break and the wildlife in the marine environment was something to be aware of, McKenzie said.

"But as I say this is the first one I've known of, I don't know the history in years gone by, but I don't think there have been any shark attacks in this area. One out of the blue you'd hope."

He hoped it would not scare people away from their usual holiday spot.

Main image (Lisa Collins/Robert Harding Heritage via AFP): Sevengill sharks are up to three metres in size and can be found all around New Zealand in shallow to deep waters (file image).