Stephen Clearwater, of Lumsden, was fishing on Monday afternoon with family and friends from a 4m aluminium dinghy near Dead Man Beach at the entrance to Halfmoon Bay.
As he pulled in a fishing line, a great white shark lurched out of the water and took the fish, ramming the back of the dinghy with its open mouth, denting the boat.
Mr Clearwater estimated the shark was about 3.5m long.
''I didn't see it until the fish was at the back of the boat, and the fish and the shark arrived all at once. It came out of the water with its mouth open, took the fish and gave the back of the dinghy a shunt,'' he said.
''It did a slow circle around the boat, with its fin out of the water, then disappeared, and we didn't stick around either, although we stopped and told some divers about it.''
Mr Clearwater's grandson, niece, a friend and his brother-in-law were with him in the dinghy.
He said it looked as if fishermen had been gutting fish in the area, which might have attracted the shark.
''It was quite scary. I didn't think I would be that scared about seeing a shark.''
Mr Clearwater regularly visited Stewart Island and said stories of shark encounters there were common.
Margaret Hopkins, of Aurora Charters, who rented the dinghy to Mr Clearwater, said the number of shark encounters with recreational and commercial fishers was increasing.
It was becoming more common for sharks to approach, and even attack, small boats around Stewart Island, particularly near where cage divers operated, she said.
''People in small boats out there have sharks coming up and grabbing lines and attacking boats.''
Mrs Hopkins did not think it was a good idea to change the natural behaviour of wild animals, and believed sharks were now equating boats with food, in part because of cage diving.
Fellow Stewart Island resident and fishing charter operator Brett Hamilton, of Southern Limits, also said sharks were becoming less timid around boats and humans, which they now associated with food.
''We are worried about the change of behaviour in the sharks. They are coming into contact with more boats and humans and they're getting used to it, so they are not as timid any more,'' he said.
''They associate the sounds of boats and the presence of humans with the presence of food or the opportunity for food.''
A retired commercial diver and long-term Stewart Island fisherman, who was reluctant to comment publicly, agreed.
''The sharks have always been here but they've always been very timid. They've done their thing and we've done ours. Now, they're less scared,'' he said.
''They've been taught that boats equal food and they've been trained to go to boats.''
Mike Haines, of Bluff-based cage diving operation Shark Experience, disputed such claims.
He said sharks were ''not at all'' becoming more familiar with boats.
''I've been diving around here for 30 years and sharks have always come up to the boat every now and then,'' he said.
Shark Dive New Zealand director Peter Scott, of Dunedin, felt the same way.
He said sharks around Stewart Island had been interacting with commercial fishermen since 1886 and nothing had changed.
''Seal numbers have exploded so there's plenty of food for them. They don't need to be chasing fish.
''There may be more sharks now that they're protected, but I haven't noticed any difference, and the number of encounters is pretty much the same as when we arrived,'' he said.
Former Stewart Island resident and PauaMac5 chairman Storm Stanley, of Blenheim, said he and others in the paua diving industry were ''deeply concerned'' about changing shark behaviour near Stewart Island.
It was a matter of when, not if, there was a fatal shark encounter there, he said.
The Government was remiss not to address concerns about cage diving and changing shark behaviour before things got worse, he said.
