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A great white shark circled a fisherman's boat for half an hour at the Tauranga Harbour entrance - sparking a warning for other water-users to beware.

Retired construction supervisor Alastair Blair snapped the 3m-long predator on his cellphone camera after it approached the boat he and a friend were fishing from on Sunday morning just off Matakana Island.

"We had burley out and he came in with the burley," Mr Blair told the Bay of Plenty Times.

The shark began circling the 5m aluminium boat and tapping the motor with its body.

"He must have been around the boat for about half an hour."

Mr Blair was awe-struck.

"We were quite excited. It's not often you see a white pointer [great white shark] like that. I've been out there for 30 years [fishing] and have never seen a white pointer," he said.

Great whites usually roam around Stewart Island, the Chatham Islands and areas with large seal colonies - a key chunk of a shark's diet.

Mr Blair was at the north entrance to Tauranga Harbour at a depth of about 15m when he saw the shark and he expressed concern for surfers who frequent Matakana Island.

"There are people who go surfing at the Matakana bar and there's a shark out there. Maybe he's gone, maybe he's not.

"But I just want people to know there's a white pointer hanging around."

Department of Conservation marine scientist Clinton Duffy confirmed Mr Blair's pictures were of a great white.

Mr Duffy said although great white sightings were infrequent in the Western Bay of Plenty, this one was not odd.

"The Tauranga Harbour entrance does not surprise me because harbour entrances like that often have a high concentration of food. You have schools of snapper and fish spawning there, which attract bigger predators like sharks."

Also, October to March is considered "shark season".

Mr Duffy said great whites were migrational and moved around the country a lot, travelling up to 100km a day.

However, if the feeding was good it was likely the shark would stay in the area.

Mr Duffy said there was no reason for people to be overly alarmed and he often heard of sharks appearing when fishermen brought in their burley.

So far this year there had been 13 reported sightings in New Zealand that he was aware of "but that will be greatly understated, I suspect".

Top national surfing competitor Tim O'Connor said the sighting would not put him off.

"If someone saw a shark it could be hanging about or it could be just cruising but it's not going to stop me from going for a surf."

Mr O'Connor said Matakana Island was one of the best beaches in New Zealand to surf from on a good day.

* Divers: Stay on the sea floor and keep an eye on the shark until it goes away. If you need to come up, try to do so directly under the boat or swim to shore.

* Surfers: Get out of the water immediately with as little commotion as possible.

*Free divers or swimmers: Swim directly at the shark, as this is seen as aggressive behaviour and the shark will probably be scared off.

*Boaties: Bring in any burley. Department of Conservation

- Bay of Plenty Times