Red Boats charter skipper Andrew Somers believes he may have witnessed the incident on a return trip from the Riverhead historic tavern on Monday afternoon.
"It jumped high out of the water three or four times before obviously snapping the fisherman's line," Mr Somers said. "It was hard to tell from a distance but it definitely would have been something like a great white or a mako, just by the way it tail-walked. Generally, schooling sharks don't do the big jumps and dives like that."
Another charter skipper, Ray Phillips of NZ Fishing Safaris, said he doubted the shark was a great white.
"I'd put my money on a bronze whaler, but who knows?" he said.
In 15 years as a full-time charter skipper he had seen plenty of great whites on the West Coast and at the Chatham Islands but never on New Zealand's east coast.
Mr Duffy said great whites were more common in west coast harbours such as the Manukau and Kaipara, but the species had historically been common in the Waitemata.
"Anything up to about 3m and it could have been a large bronze whaler. But the fact that it breached - jumped clear of the surface - is typical white shark behaviour, particularly when you hook them in shallow water.
"People are pretty quick to dismiss sightings of great whites but there have been several sighted in the outer Gulf over the last month and this is not unusual habitat in my experience of looking for them and trying to tag them."
There had been no further sightings reported but Mr Duffy urged people to exercise caution.
"We advise people who are fishing, kayaking, or kite surfing in this area of the Waitemata to keep an eye out for this great white, to take special care and keep themselves safe."
