Australian-born writer/director P.J. Hogan’s last film was 2009’s Jerry Bruckheimer-produced Confessions of a Shopaholic, the sort of cut-and-dry American rom-com audiences quickly forget and critics don’t think twice about sinking the boot into.

Mental, set in a fictitious Australian beachside suburb named Dolphin Heads, reunites Hogan with star Toni Collette (who helped set his career in motion with Muriel’s Wedding [1994]) and frees him from the commercial constraints of a Hollywood studio picture. Hogan responds by swinging wildly the other way, directing a screwy self-written screenplay (inspired by his own upbringing) hellbent on playing chasey with conventions and audience expectations.

Shirley Moochmore (Rebecca Gibney) is a sweet, batty, cuddly mother of five young girls and wife to philandering mayor Barry (Anthony LaPaglia), who is loved but almost always absent from the dinner table. The film kicks off with Shirley prancing around the backyard singing The Hills Are Alive. Before long she’s whisked off to a mental asylum and Barry begins a campaign to get re-elected.

The young’uns are convinced they have mental disorders too. The eldest (Lily Sullivan) ‘attempted suicide’ by jumping off the garage roof and landing, plomp, on dad’s car as it pulled into the driveway. Another has recurring nightmares featuring Rorschach-like Lost in Space creatures.

Driving home one afternoon, Barry picks up bong-smoking, knife-wielding hitchhiker Shaz (Toni Collette) and without thinking twice enlists her as a live-in babysitter. Rough and lippy, with a loyal pooch named Ripper, Shaz shakes up the Moochmore’s calm well-kept neighbourhood. She enrages the local populace and takes on a school-of-hard-knocks mentor role for the kids, upturning assumptions of themselves as social pariahs.

What Hogan is broadly getting at is obvious early on — a riff on the old adage that to the crazy person the normal person is insane, and having a good heart is more important than keeping a tidy lawn. He flips that adage backwards and forwards: crazy is normal and normal is crazy and crazy is normal, and normal is crazy, and crazy is normal, and yep — we geddit. The best touch is his warts and all rendering of Shaz, the knowledge that the person leading the charge through dense mental wilderness might be the kookiest — or worse, the least reliable, or even the most damaging — of them all.

“I can’t tell whether everything is coming together or falling apart,” Shirley says from inside the funny farm at the tipping point of the final act, if you can call it that. It’s more a curly collection of call backs to previous scenes, expanding jokes and extending thought bubbles, some of which never flew that well or lingered memorably in the first place. That line may echo the thoughts of viewers who don’t get into the film’s bouncy, ridiculous vibe, who don’t see the beauty of it, the way the character of the film mirrors the characters in it: Mental is mental too, temperamental and unconventional in bizarre and funny ways.

“Mental is mental too, temperamental and unconventional in bizarre and funny ways”

It’s a family film unhinged from the expectations that label draws. Hogan keeps hurling potential taboos at audiences with gleeful licking-lips abandon. There’s DIY diagnosis of mental disorders, suicidal tendencies, a splash of drugs and toilet humour, use of the “c” word, a sex joke involving a dog. Even a scene in which menstruating women leak and smear blood on bleach-white couches and walls of an obsessive compulsive cleaner. A lot of the time, in Australian comedies, this sort of material is cringe-worthy. Here it’s actually funny. When it’s not it’s suitably strange.

These potential taboos are rarely lingered on. Mental  is not a sweary movie; it’s not a gross-out comedy; it’s not in any way a safe family drama. Yet it’s also genuinely for all ages, mingling themes of struggling family, infidelity, fractured father figures, coming out, small town prejudice, first romances, etcetera. It is a bare-all everything-in-the-open film, and the way it encourages discussion of important issues without appearing didactic or preachy is admirable.

The slippery structure won’t work for all audiences and some storytelling messiness is par for the course when you take on this much this tangentially. If viewers can’t get into the rhythm, the dramatic moments and the jokes may not connect, but the film offers enough of both.

Hogan’s impulses this time around are both populist and unconventional, as if he can’t decide whether he’s directing an indie or a multiplexer. That’s partly why Mental is so strongly unique, so difficult to contextualise, so easy to herald as a triumph of ballsy suburban dramedy or an ambitious dud. The truth lies somewhere in the middle, erring towards the former.

The film is speckled with strong performances, particularly from Collette. Once again she is a living lightshow of expression, charismatic and all-inhabiting. Gibney, whose potentially “look, I’m acting crazy” presence is offset with sugarplum maternalism, is very good too, and import Liev Schreiber as a rough-as-guts shark hunter sets a gold standard for Americans impersonating an Australian accent. He’s bang-on.

A bunch of fake endings are crucial to Mental’s crazy-in-the-coconut tone, and Hogan knows he’s melting plotlines and stirring up expectations. He uses them to unravel his own story resolutions, which gives the film the curious feeling that progress has been made, of a kind, and it’s good, sort of, and bad, sort of, and normal and insane and insane and normal, and nobody can really say what is right or true or just or appropriate. Yes, yes, yes: the film is a bit crazy. In a good way.

Mental’s Australian theatrical release date: October 4, 2012.