Emergency staff confirmed the person's body had been pulled from the water following the early afternoon attack. The shark had been killed but was still floating in the ocean. 

Wellington student, Stef McCallum, 18, said they were first aware something was wrong when a man ran across to the surf club to get help.

A woman told us a man had just been killed by a shark, she said.

"She said there was a big pool of blood in the water."

Mr McCallum said they saw a police officer go out in a surf boat and shoot the shark.

"He fired about 20 shots."

About 200 people were on the beach and people quickly ran.

"Everybody was evacuated from the water. Word of mouth, 'shark', and everybody left the water."

TV3 reported from the beach that as many as three sharks may have been involved.

A witness told the TV station he spotted the lone swimmer while he was out fishing at the beach, shortly before 2pm.

The distressed man signalled for help when he was attacked, before he was pulled underwater.

At this point, the witness said three or four other sharks appeared in the area.

A staff member at Sand Dunz dairy said people were coming into the dairy "speechless".

"I'm shaking, I don't really want to talk about it," the shop assistant, who didn't want to be named, said. "Everyone's speechless."

Earlier this week surfer Bourne Nobel Buiski posted on Facebook that there had been a "massive" shark spotted near surfers on Monday at Piha, 14 kilometres south of Muriwai.

He said that a local man ran out of the water "white faced and terrified".

"He was saying that a great white, a massive great white had just swum right beside him,"  Buiski said.\

No one believed him, he said.

"As they are so rare here ... There were about 60 people there, and no one came in."

The beach was crowded with children from Glen Eden Intermediate and Avondale College, a Fairfax reporter on the scene said.

Auckland City Council said the beach had been closed for the day.

There have been 14 known fatal shark attacks in New Zealand, since records began about 1837, according to Department of Conservation shark expert, Clinton Duffy.

"In the last 20 years we have been averaging two shark incidents, where the shark actually bites someone, a year. Those are generally on swimmers and generally result in fairly superficial flesh wounds," Duffy said.

The last death was in 2006, when a kayaker was mauled by a great white in the Coromandel - whether he drowned before the shark found him is still disputed. Before that the last death was in 1976.

Global shark attacks have increased every decade since 1900. Last year's 12 fatalities, three in Australia, was almost three times the average of 4.3 from 2001 to 2010, according to the International Shark Attack File.

There are more than 60 shark species that come to New Zealand waters. The majority are little-known species that live deep below the ocean surface.

Fairfax NZ News