Police are urging caution in the water after a woman was bitten by a shark in an estuary recently.

The 21-year-old received a significant laceration to her leg and had been walking in knee deep water.

Police believe the culprit was a seven gill shark and that the time of day may have been a factor.

Shark scientist Riley Elliott says people need to be aware of their surrounding and need to be mindful when fishing.

It was common for seven gill sharks to be in the estuary at this time of the year - coming into the shallow water just as their prey does.

Being dark at 2am, the woman would not have been able to see what was there, he says.

"This is when sharks, like seven gills, capitalise on being able to use their senses other than vision to catch prey that are mainly visually avoiding predators.

"When it's summer and we go swimming in the ocean there are predators out there."

He says there are not generally specific areas around the country that are more dangerous.

"I mean the ocean is full of fish, they're widely distributed and the species vary."

In the North Island the bronze whaler is the most commonly encountered shark. 

"But they're very attuned to people and generally avoid us."

Seven gills are very common in the South Island.

"They're called a cow shark around the rest of the world because they're kind of docile like cows, they move very very slowly, very non-active but if you put food in the water they can turn on quite quickly and become quite bitey," says Elliott.

"Sharks don't have hands so they investigate with their mouth and you can imagine a shark in the darkness navigating on vibration and splashing and smell and all of a sudden there's a white piece of flesh in front of you and you might think it's your natural prey so you have a bite."

People need to be aware of their surroundings, he says.

This morning Riley was holding his newborn baby at the beach while his wife was paddling out to surf.

"Literally a metre from the shore there was a little shore break and a bronze whaler was swimming through the waves."

If people were fishing in the area or had dumped fish carcasses, it would have been a different scenario, says Riley .

"Situations like that are ones people should avoid are things overlapping with the presence of fish or fishing because that is what can catalyse a shark to get opportunistic or take a bit at something it might not 100 percent confirm what it is."

If someone does encounter a shark they should be calm in the water and retreat, says Riley.

"It's just a respectful thing to do with any wild animal that you don't understand.

"But learning more about them, understanding where they are, what they do, and avoiding potential overlap with their food is the best situation because at the end of the day statistics speak for themselves because sharks very rarely have an adverse interaction with people and it's generally always because of actions that we're doing or unknowingly overlapping with like the presence of a bunch of fish carcasses that have been dumped."

"My advice would be bury your fish carcasses in your garden - it's very good for it. Dispose of them away from where people swim."

Swimmers need to be aware where they swim.

The biggest risk in the ocean was drowning, he says.

"Manage your largest risk which is the ocean itself, by swimming between the flags, wearing a lifejacket and getting a Coastguard membership."

-RNZ.