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Father-of-one Adam Strange (46) was attacked by the shark, measuring at least 3m, just before 1.30pm.
Mr Strange, a television commercial director, was seen waving for help before other beachgoers realised what was happening in the water and called 111.
The New Zealand Herald has learned that police received multiple calls, the first from a man on the rocks at the south end of Muriwai Beach.
A police source said it was possible up to three sharks, most likely great whites, had been feeding on fish and birds, and Mr Strange swam right into the middle of them.
The police Eagle helicopter spotted the shark while it was still attacking Mr Strange and stayed above until officers got to the beach.
One was sent out on the water in an inflatable rescue boat with three lifeguards.
It is understood that when the inflatable got to Mr Strange, the shark still had him in its grip. He was already dead. The officer used a M4 Bushmaster rifle to shoot at the shark to get it to release the body.
Mr Strange's body was pulled into the inflatable and taken to shore, where his wife, Meg, and other family were waiting.
Just before the attack, several hundred pupils of Glen Eden Intermediate were swimming nearby.
"Just after we got out of the water, the Westpac helicopter came over and then we saw the policeman get into the IRB," parent helper Tracy Howarth said last night.
Adam and Meg Strange married in 2011 and have a daughter, Indigo, 2.
Mrs Strange was not ready last night to speak about her husband's death. But a family friend gave a brief statement to the Herald.
"The family are grieving the loss of a glorious and great father, husband and friend. We are in deep shock and are still trying to contact family who are overseas," he said.
Mr Strange's wife was being comforted last night by friends from the tight-knit Muriwai community, who converged on the house after learning of the tragedy. Many took flowers as expressions of their grief and their solidarity with the family.
On his website, Mr Strange said one of his short films, Aphrodite's Farm, set on a dairy farm in the 1930s, had been in 10 film festivals and last year won the Crystal Bear award for Best Short Film at the Berlin Film Festival.
He was also a finalist in the global Cannes Lions International Advertising Festival and the London International Awards.
Mr Strange began making commercials in 1995 and was a director for Silver Screen Productions, in Auckland, for more than 10 years.
His work took him to Japan, China, Thailand, Malaysia, the United States and Europe.
In his biography, he described his love of the outdoors and spending time with his family.
"When I get a spare five minutes, I like to make a fruit smoothy, surf some big waves out on the west coast, point my skis down a mountain with Meg, haul my mountain bike up and down a few hills, drink some Pinot while scratching away at a film script."
The fatal attack was witnessed by a number of beach-goers.
Pio Mose was fishing with a group of men on the rocks when he saw the "huge" shark attack Mr Strange just 50m away. "All of a sudden there was blood everywhere."
Mr Mose said he saw Mr Strange struggle with the shark before it swam away. He was keeping his head above the water before the shark returned. "I yelled at him to swim to the rocks."
Mr Mose watched helplessly as the shark took Mr Strange's body out to sea, and when lifeguards arrived, he directed them to the group of sharks.
"It's awful - it's scary like a nightmare to me. All I was thinking was I wanted to jump in the water and help but I didn't want to get attacked by a shark too."
A resident who lives above Maori Bay saw the drama unfolding.
"Then I saw the IRB on the water and heard the explosions and saw water flying up. They were shooting at the shark in an effort to get it to release the body.
"There would have been up to 10 to a dozen shots fired - some in very quick succession."
Beaches on Auckland's west coast are closed until further notice.
- Anna Leask and Andrew Koubaridis/additional reporting: Amelia Wade and Kieran Campbell