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The sun is rising over Sydney's Bondi Beach and in the pre-dawn cool, about 30 people are making their way through a set of arduous exercises -- tossing weights onto the sand, doing chin-ups and flexing an enormous rope up and down.

The group are part of one of the fastest-growing sectors in the fitness market -- boot camp -- where the confines of the gym and private, individual exercise are shunned in favour of training with a group in the great outdoors, often being pushed by a military-style trainer to a high-intensity workout.

"I think I am never going to do this again. I throw up. I hate every minute of it... but the satisfaction... there's nothing more rewarding," explains Kristie Webster after her session at Bondi.

So why does the 26-year-old drag herself out of bed in the dark several times a week, knowing that she's in for such physical discomfort? "Because I'm insane? No, just purely for the fact to get fit, to get healthy."

"We push them as hard as they can be pushed, but we do it in a motivational way. We're not like a drill sergeant, we're there to sort of encourage them," explains her trainer Dan Clay, who is originally from Essex in Britain.

Clay, who runs the Dangerously Fit company, has seen the popularity of boot camps skyrocket in Sydney in the past five years, as they have elsewhere in the world.

"People don't want to always be stuck in a gym. They want to come down first thing in the morning, they get the beach, they get the sunrise, they get the fresh air. And it's good for the soul, it's good for the body, so it's got really popular," he says.

Training in a group provides additional motivation, he adds.

"People want to get outdoors and train with their friends," he says simply.

Without the costs of major chains and few overheads, boot camps have sprung up around Australia, while other physical challenges such as Tough Mudder -- hardcore 10-12 mile obstacle courses which can include electric shocks, steep cliffs and walls, and fire -- have also proven popular.

James 'Chief' Brabon, the former Australian army infantry soldier behind Sydney's Original Bootcamp, is considered a pioneer of the industry.

After initially developing his training regime for fellow soldiers, and then their impressed wives and girlfriends, he eventually re-established the brand with gusto in 2003, using one of Bondi's near neighbours, Coogee Beach.

He said he struck scepticism at first, particularly given the trend in fitness at the time towards more gentle exercise such as yoga and pilates.

"And then we came out with this disciplined, high-intensity sweat-till-you-drop sort of training, and people were saying 'No, it won't work. Won't work, won't last, nobody wants to be told what to do'. And what we've found, is that everybody wants to be told what to do," he says.

"We don't believe in berating people, and demeaning them, and yelling and screaming -- no one would come back. But basically they want to just be guided. They want to know they are doing the exercises the right way, doing the right type of training."

'Chief', who teaches five mornings a week, says the military element is just an excuse to be able to push people to a higher intensity.

"It's not play acting so much as window dressing," he says. "It's very high intensity sports-style training and we use military-style overtones to help strip away a lot of the excuses that people have.

"I suppose we're seen as controversial in our industry because we don't believe just moving is enough. We've staked our whole reputation on the fact that you have to work. There's no use just turning up. If you don't challenge yourself you will not improve."

Back in Bondi, the group are happy to have had the tough workout -- an activity which is at odds with Australia's growing problem with obesity.

"The pain can sometimes be great but it's always worth it at the end, you push yourself because you know it's only 45 minutes," says Stafford Hamilton, who credits Dangerously Fit with helping him shed more than 11 kilograms (24 pounds) since he started nine months ago.

"And afterwards you feel fantastic. It's worth it, it's worth the pain," the 43-year-old says.