An increase in shark sightings around Stewart Island is fuelling debate over whether caged shark diving is changing animal behaviour in the area.

Brett Hamilton, a commercial fisherman and owner operator of fishing charter Southern Limits, said three great white sharks had been sighted in a bay on Saturday, something he considered highly unusual.

"I've been fishing around here all my life, I turn 50 next year, and we hardly ever saw a shark and now we see them all the time, like, all the time."

Mr Hamilton believed the increase in sightings was down to shark-diving operator practices changing the behaviour of the animals. "They're teaching a shark to get a feed from the boats."

"They dangle a big piece of tuna over the side of the boat to make them come in, if that's not changing the behaviour of an animal I don't know what is."

However, shark-diving operators say their businesses were not having an impact on shark behaviour.

Shark Dive New Zealand director Peter Scott, of Dunedin, said the place where he believed the commercial fishermen were seeing the sharks was the same place where commercial fishermen were cleaning their catch, so it followed that there were sharks looking for easy food.

Conservation efforts worldwide, with the official protection of the species, had been paramount in increasing the numbers of Great Whites too, he said.

"The sharks have been living at Stewart Island for a lot longer than humans."

Shark Dive NZ was yet to start the season yet so it didn't follow that the company had brought sharks to the bay, Mr Scott said.

"We do, however, bring in many tourists, who all stay at the island, dine at the island, and partake in other tourist activities during their stay. We intend to continue to bring tourists in to boost the island's economy in the foreseeable future."

Mike Haines, of Bluff cage-diving business Shark Experience, denied the claims cage diving lured more sharks to the area.

"It's nonsense and that's that," he said. He did not wish to comment further.

However, Stewart Island resident and former commercial fisherman Richard "Squizzy" Squires disagreed with operators, saying the increase in sharks only happened once cage shark-diving started up.

"I've been commercial fishing for quite some time and I have never seen in my life a [live] white pointer until they started shark cage-diving."

Sharks were becoming more bold as they associated boats with food, he said.

"I see both sides of the coin. These guys have got to make a living and I understand that. To be honest, I've taken the boat out beside them while they're doing it and they've [his passengers] got a real thrill out of it."

But the relationship between the activity and the increase in sharks was undeniable, he said. "I've had one attack my boat before. In the last five or six years we see them all the time. Now they're getting more bold."