The shark appeared to be a 3m great white, Mr Walsh said.
"By the time I looked back and saw that, Kevin was already under the water, so I didn't see him in the mouth of the shark, but I saw from the front of the gills almost to the tail had come out and hit him and had already started going back into the water, but twisting at the same time."
"He was sitting on his board and it got him on his thigh. It pinned him to the board as it took him down.
"When I called Kevin later after surgery he said he thought he'd been hit by a line-backer. He said all he remembers is the impact and then being under the water."
After the attack, Mr Walsh said his friend "sprint paddled" back to shore. "We were about 200m off the beach, he had a full adrenaline rush and sprinted to the beach and I was obviously behind him, but I couldn't keep up."
When he made it back to shore Mr Swanson had already used his surfboard's leg rope as a tourniquet to stem the bleeding, Mr Walsh said.
"By then some other folks had seen what happened and [came down]. We dragged him from the water up on the sand."
Fortunately a doctor was on the beach and was able to assess Mr Swanson's bite wound, he said.
"She decided there didn't appear to be any major bleeding from arteries, so that allowed us to realise the situation and know he wasn't going to bleed out then and there.
Mr Swanson was flown by helicopter to hospital. He was soon released, a spokesman for the medical centre said, and is expected to make a full recovery.
"It's going to be a while, but [he's] really blessed, it could have been so much worse."
Mr Walsh, a director of a Californian produce company, said he had been living in America since he arrived as a 24-year-old in 1986.
"I originally grew up in Papatoetoe, I went to De La Salle College in Mangere East so I'm a South Auckland boy."
He said the attack hadn't put him off surfing. "I've been surfing a long time, I'll be interested to see how I feel going out, but of course I will go out.
"It's a big motivation for me, many life decisions have been based around surfing although maybe I'll be mountain biking the next couple of days."
Butchered bronze whaler left in shallows
Nicholas Jones
A kayaker was shocked to find the de-finned carcass of a large shark left to rot near a recreational fishing net.
The 2m bronze whaler died after being caught in the net put out on the western shore of Manukau Harbour.
The de-finned shark at Fletcher Bay. Photo/ Jason Oxenham
Adam Mcintosh, 45, discovered it in the shallows while kayak fishing on Sunday in the bays east of Huia. "They had just got a knife ... and cut the jaw out and de-finned it. I was quite shocked. It's a beautiful fish, and they just left it."
Mr Mcintosh took photos and sent them to a Ministry for Primary Industries officer. But after checking, the ministry confirmed the shark was caught about 100m outside of the net ban zone.
Mr Mcintosh said that, despite being legal, the catch was upsetting. The rotting carcass was still there when the Herald walked around the cliff line to Fletcher Bay at low tide yesterday.
Its fins and tail were cut off and its jaw removed. The body of a small pup shark was next to it, and a net stored in boxes on the edge of a property nearby.
A woman who lived there, who declined to be named so as to not be "vilified", said the shark had been caught in their nets, which were set legally.
Her family had put nets out for several years and this was the first time a shark had been caught. They did not want to eat the bronze whaler's flesh, but took the fins and jaw at the request of another family.
The usual catch was only one or two small fish, and the woman said a much bigger impact on fish stocks were the many commercial boats that worked the harbour. Department of Conservation shark biologist Clint Duffy said commercial fishermen primarily targeted rig sharks but were allowed to land other species as bycatch.
"Bronze whaler is quite palatable, so smaller bronze whalers end up as fish and chips as well."
