Hungry hammerhead sharks are just one of the hazards faced by the crews of Sydney's elite emergency rescue helicopters. 'We were doing a training exercise and we'd just put a man in the water when the pilot saw this big black shape circling below,' said Peter Yates, 44, who has been flying helicopters for nearly 20 years.
'It was an 8-foot-long (2.4-metre) hammerhead. Luckily we had a guy on a jet bike nearby, who was able to come to the rescue.'
The two choppers, which are based at Sydney airport, are on 24-hour standby to rescue windsurfers blown out to sea, abseilers stuck on cliff faces, bushwalkers bitten by venomous snakes and, in a recent case, a mountain biker who injured himself after colliding with a wallaby.
The helicopters have hovered perilously close to updrafts to rescue fishermen trapped in sea caves, swooped low over swollen rivers to pluck stranded motorists from their cars, and patrolled far out to sea in search of lone fishermen delirious with sunburn and thirst.
'Sydney is surrounded by national parks on three sides and the sea on the fourth,' said Trevor Cracknell, a crewman with 20 years' experience. 'You have 4 million people with easy access to all these recreational activities. There are lots of ways of getting into trouble.' The most gruesome rescue jobs are those involving motor vehicle accidents. The helicopters pick up the critically injured and take them to the nearest hospital. Some patients are so badly maimed that they have to be operated on in the cramped confines of the helicopter.
'It can be a bit of an abattoir in the back,' said Tony Wood, 49, a former British Royal Marine who saw action in the Falklands, Bosnia and the Gulf. 'The doctors have performed amputations while we're still in the air.'
Wearing a khaki fire-retardant flight suit and a helmet with a visor, the Englishman crouches in the open doorway of the Kawasaki chopper as we swoop over Sydney's Botany Bay, site of Captain Cook's landing in 1770. It's a beautiful day, and I scan the turquoise shallows and pale sandbanks for those ominous black shadows.
There's nothing today. But they're out there. 'We see packs of 60 or 70 hammerheads,' Mr Wood said.
The rescue service, which started in 1973, is operated by Surf Life Saving Australia with modest sponsorship from a major Australian bank. The rest of the operating costs are met by donations from the public - including some of those who have been rescued.
'Some people are incredibly grateful and send you a crate of beer,' Mr Wood said. 'Then you have other people who go all silent because they know they've screwed up, and they're really embarrassed.'