Occasionally, an inside story can come from too far inside, and “Bra Boys,” which opens Friday at the Quad Cinema, is a good example. This Australian documentary about a controversial surf gang in a Sydney beach slum is directed by a member of the featured surfer clan, and the result is essentially a promotional film. Despite the indisputable wonder of watching sun gods glide through barreling waves, it’s hard to tell in this muddled account where blithe bias ends and sloppy technique begins.

A history of Maroubra Beach introduces the titular tribe of rough-and-tumble surfers with a level of context that is rapidly abandoned. This woe-begotten “inner-suburb,” a 19th-century melting pot that was slum-ridden by the 1930s, is overlooked by a prison and abutted by a sewage plant and a rifle range. But at least, as someone says, it’s by the beach, an antipodean fact of life that engendered successive maritime gangs and culminated with the Bra Boys.

Before riding waves and knocking heads, the hundred-strong Bra Boys incubated under the tireless care of a beloved den mother — actually, a grandmother — of the fatherless Abberton boys. Community, loyalty, and (maybe the key word) fraternity are the movie’s refrains, and the Abbertons — three brothers who raised one another — seem to be the group’s core. The buff, bluff, shirtless Bra Boys have one another’s back at the worst of times, and the best. Koby Abberton, an internationally known and corporate-sponsored surfer, appears almost as a Bra spokesman.

But it’s the worst times that have stigmatized the Bra Boys, who here get special pleading as misunderstood brawlers. Though self-identifying with their seaborne sport, the Bra Boys have also fought off gang attacks since the ’90s and notoriously crashed an off-duty police Christmas party in 2002. In addition to footage of bloody, bare-fist beat-downs, we see some of the group’s “Jackass”- like antics, which the filmmakers balance with their mentorship of aspiring teenage surfers.

While this dual lifestyle, which is bewilderingly full of Aussie good cheer, could make a fascinating portrait, “Bra Boys” is not up to the complicated task, and is disappointingly willing to explain away almost anything. Then, halfway through, director Sunny Abberton abruptly drops the overview to feature thrash-metal-scored surf highlights and a murder trial implicating his brothers. This is the ethos in a nutshell: Extreme adversity requires extreme measures that make men capable of extreme sports.

Key details about the case on trial, which was considering brother Jai’s alleged murder of a neighborhood psycho, are absent, even as “Bra Boys” milks the expectation of a verdict for suspense and family honor. After Koby is charged as an accessory for refusing to supply information to the police, his globe-trotting surfathon, which occurs between trial dates, serves to pad out the film. For a stirring conclusion, the film plays up the gang’s peacekeeping in the aftermath of 2005 race riots that attracted muddled press coverage. Colorful historical detail (including a sneaky law requiring surfers to wear skirts) and mopup during the credits (a multiethnic roll call of beach denizens) hint at a better “Bra Boys.” And serviceable portraits of subcultures from someone deep within the scene are not impossible. The 2001 Southern Californian skatehead documentary “Dogtown and Z-Boys” was directed by pioneer skater Stacy Peralta, but was assembled well enough that few seemed to mind.

“Dogtown” also inspired a fiction drama (2003’s “Lords of Dogtown”), which may be the fate of “Bra Boys.” Russell Crowe, the star-on-low-power narrator for the documentary, has been reported as the director attached to its feature version (his debut in that role). Perhaps the remake will succeed where this curious mixture of bonhomie, brawling surfers, and on-message friendly-gang pamphleteering fails.