Who can understand the passions of others? It's Saturday night at Sydney's Luna Park, and the loudest sound in the Big Top - louder even than the cheering of the sell-out crowd - is the heavy slap of bare flesh hitting polished concrete. On a small track, 10 women in fishnets and five kinds of protective body gear are rollerskating round a ring, sometimes alone, but mostly in one tangled, sweating group. On almost every revolution at least one girl crashes to the ground, sequins twinkling as the air slams out of her lungs.

To the untutored eye, the whole thing looks like some kind of crazy burlesque show, crossed with an especially violent cage fight. But this, apparently, is a misconception. "As a friend once said, roller derby is like speed chess," says a tall blonde skater, whose derby name is Annabelle Lecter. "While someone throws bricks at you."

Everyone I talk to about roller derby is totally obsessed by it. Indeed, one suspects obsession may be almost a prerequisite for the game, given that it's absolutely amateur (even in America, where it originated); that it's organised and administered entirely by volunteers; and that it demands enormous amounts of time, energy, and determination to actually play. You ideally need 14 officials for a match - seven on-track referees and seven score-keepers and adjudicators - not to mention roller skates, a track, a mouthguard, a helmet, wrist-, knee- and elbow-guards, and 10 girls who understand the rules. These are incredibly complex, and involve two teams, each with five players, one of whom, called the "jammer", is responsible for scoring: one point for every opposition player she passes. Everybody else simultaneously defends and attacks to either prevent or promote this passing, and there are all sorts of meticulous regulations governing play.

"There are layers upon layers of strategy," one girl tells me earnestly, pulling up her knee socks. "There's strategy I can't even see." This is certainly true: what I can see, basically, is 10 girls, bruises blooming, false eyelashes falling, knocking each other over.

Of course, such a spectacle is not without visual appeal. The game can be traced to the 1880s (before either NRL or AFL were formally organised in this country), when official roller skating races with cash prizes were being held in the US, and people queued up to watch the contestants ram into each other. By the 1940s, the sport had become a recognisable version of the modern game, and by 1948, bouts were broadcast via the miraculous new medium of television, while week-long contests sold out New York's Madison Square Garden.

This popularity endured for the next decade, but slowly, in an effort to maintain TV audiences, roller derby bosses began to artificially adjust the game's natural dramas. Players were marketed as heroes or villains, storylines were scripted, and outcomes of bouts were predetermined. By the 1970s, for better or worse, roller derby had become a form of sports entertainment, a lot like World Wrestling. Think Hulk Hogan; think Spandex, glitter and Raquel Welch (in the 1972 movie Kansas City Bomber) in a jumpsuit.

And just like Hulk Hogan's gold lamé, the gloss eventually wore off. These days, as roller derby players point out, competitive trampolining is more likely to get on television than they are. In the early 2000s, the sport was revived as a volunteer-run, all-girl operation, and it's subsequently become a pin-up pastime for feminists and right-on socio-cultural analysts. As one recent academic text puts it, "Roller derby has emerged as a gendered space for self-transformation, belonging and embodied contest."

Thrilling as this sounds, most women don't mention embodied contest as a reason for loving roller derby. They mention the fun of dressing up - how often does the average woman get to wear a stage-make-up cutlass slash and call herself Pain Fonda or Princess Slayer, after all? And they mention the constant, blood-tingling threat of serious bodily injury.

Phar Slap - real name Jasmine Still - first heard about roller derby from her twin brother. "He was doing the first aid for a league down in Canberra, and he told me about this thing where girls hit each other on skates," she explains, sounding awed. "And about how this girl got this amazing spiral leg fracture. And then I got some $10 skates at a garage sale and I thought, 'I could do this thing.' "

Like many, many roller derby players, Still appears to have two discrete lives. By day, she's a lawyer, and by night she's Phar Slap, long hair flying, long limbs sliding around opposition players as they plummet to the floor. How can someone who looks so much like Emily Dickinson contain so much violence? And how does she manage her alter ego during her day job?

She gives a little laugh, hunching her shoulders. "It's a little like Superman," she says. "He takes his suit off and underneath, he's someone entirely different. It's a relief to come here. I've never really played sport before, and this is really physical, and a little violent!" She laughs again, then adds hurriedly: "In a controlled, safe way, of course! It shocks people, the first time they get hit. But strangely, I was like, 'Yay!' " She still remembers her first training session: "When I saw my first training game, they were talking about rules, and I was like, 'This thing has rules?' "

Not that the rules seem to protect anyone, when push comes (literally) to shove. Still sustained a 12-centimetre muscle tear in her quad last year; her fellow player, Feisty Cuffs, has broken both ankles (which have required five surgeries with one more to come), and been concussed twice.

"I went and saw this great game at Christmas four years ago," recalls Feisty Cuffs (who now wears a helmet made for professional ice hockey players). "It was called Season's Beatings, and then there was a Fresh Meat class [a kind of try-out session for new players]. I couldn't skate at all - I rented my skates for the first few sessions - and I spent most of the time falling over."

Feisty Cuffs was drawn to the game despite - or perhaps because of - having no history of athleticism. "But at Fresh Meat, even though I spent most of the time on my arse, some of the girls said to me, 'We'd like someone of your stature to join the team, so let us know if we can help you.' "

This is a common story among roller derby players: although there are a few who look like captains of the netball team, a great number seem to be the girls who, once upon a time, were wagging gym class and not getting picked for the softball team. "I've never been good at sports," confesses The Mighty Toosh (whose real name, even more magnificently, is Patricia Guadalupe Arellano Amaro). "I mean, I came up with my derby name after I went on a Mardi Gras float and I saw myself and I thought, 'Oh my God, my arse is enormous.' " She laughs. "I've always been really unco; I can't run. And you don't want to be the fat kid; the unco kid. You don't want to be the clumsy kid with Band-Aids on her knees when everybody else is being a glamorous ballerina. But you go to derby, and there's all sorts; all these different kinds of girls."

Arellano Amaro, who is 34 and the manager of a tour company, has dropped four dress sizes since she began playing. And Goldie Horns - Emma Goldie - who works for ASIC doing things like incorporating companies and writing charters, came to roller derby via a roller skating fitness class. "Some girls from that class were starting up a league," she recalls. "I just did it for training, really. I'd never been to a bout, I didn't know the first thing about it, but I just fell in love with it. The first bout I ever went to, I was totally blown away."

Goldie is 42, and is now playing with girls literally half her age, which is another benefit of roller derby: like weight, height, or conventional athleticism, age seems not to be a barrier. "Derby is my midlife crisis," says Goldie with relish: "It's much better than toy boys. And also, I have a teenage daughter, and she is just so embarrassed." She laughs. "My doing this is just the worst thing that's ever happened to her in her whole life. So I keep her in line by threatening to turn up at school in hot pants. Sometimes I punish her by bringing her to bouts. It's fantastic."

Those who love it take roller derby very, very seriously. There are more than 130 leagues in Australia, including men's, mixed, and junior teams, and there are plans for more inter-league games, more interstate games, more international games. The only Australian team regularly competing overseas, the Victorian-based VRDL All-Stars, placed fifth in their first international tournament in 2012 and is expected to do even better this year. Even the International Olympic Committee is taking notice: just last year, roller derby was under consideration for inclusion in the 2020 Tokyo Olympic Games. It didn't make the cut this time, but it shows, perhaps, the potential of the game on a world stage.

But it also reveals a paradox. Roller derby is a sport that demands extraordinary athleticism, skill and endurance: no one who's seen a bout can doubt it. But it's also a game that tries to distract you from these things, towards the glitter of its costumes, its camp, its showmanship. And perhaps, as a result, it undermines itself.

Ironically, those who love it know this. "The struggle for us is that people aren't interested in our travel team games," explains Feisty Cuffs worriedly. "Those teams are the best players of each league ... It's us playing at our peak: our elite game. But because in those games we don't wear the outfits, we don't have the theatre, people aren't interested."

And, of course, this interest is crucial. Without television deals, significant sponsorship or a big professional body to subsidise it at lower levels, roller derby has no income stream to expand its reach or ensure its future.

Added to which, many roller derby players themselves love the flamboyance of the game. "The sport is more important than the drama," concludes Arellano Amaro. "But then, the drama brings more to the sport. So for me, it's a real tug-of-war. I love the theatrics, but the theatrics are better when the sport is better."

It seems a shame they can't have both. Other sports have sequins and theatrics and variations that encompass both melodrama and Olympic-level seriousness: and if you don't want to be a gymnast or a soccer player or a wrestler, why can't you be a roller derby player in all your hot-panted glory?

Let's hope that one day, you can. And in the meantime, the passion remains. "The thought of not being able to play derby has brought me to tears," admits Arellano Amaro, waving a hand towards the track and the squeal of skates. "But I try to see life as ephemeral: it won't always be there. So every f...ing day I can do this is a good day."

Good Weekend