PJ Hogan, the Australian writer-director, must be drawn to his homeland’s female misfits. In his breakthrough Muriel’s Wedding, Toni Collette was a pudgy naïf who just didn’t fit in; now in his semi-autobiographical (his mom’s breakdown) Mental, Colette is Shaz, a wildass, highly energetic drifter packing an impressive knife, instructing her young girl charges “I’ll be the judge of that” when they tell her they are mentally ill—better that than being unpopular. Her alternative view liberates the fresh-faced, adorably unruly but emotionally stranded girls and even, eventually, their mum (Rebecca Gibney). Don McAlpine’s camera spins around the lush green hills of the imaginary seaside town of Dolphin Heads, segueing to Mental‘s most memorable scene: Gibney’s euphoric rendition of “The Sound of Music,” singing and twirling while taking down the laundry in the backyard. Every bipolar II in the audience will fly with her, though her von Trapp fantasy lands her in the local loony bin, thanks to her philandering politico husband (a funny, sometimes sheepish Anthony LaPaglia). Shaz becomes the girls’ nanny, but instead of just sitting on the couch and smoking dope, she evolves into a martinet, even forcing Dad to spend time at home, though he doesn’t recognize his daughters unless they’re wearing name tags. Mental skewers the easy-on and -off labels of psychiatry, but some sequences, particularly one of “bad dreams,” are sophomoric. The movie’s real mess-up was to move Shaz into melodrama at the movie’s end. Upenders look ridiculous when they get sappy.