Rather than re-write the plot outline, here's what the movie's website has to say:
The film follows the story of the Moochmore girls who are certain they all suffer from some kind of undiagnosed mental illness - because if they're not crazy then they're just unpopular. Their mother Shirley (Gibney), unable to cope with her demanding teenage daughters and philandering politician husband, Barry (LaPaglia), suffers a nervous breakdown. After Barry commits his wife to a mental hospital (telling his constituents that she's "on holiday") he finds himself alone with five teenage girls he barely knows. Desperate, he impulsively picks up a hitchhiker named Shaz (Collette) and installs her in his home as nanny to his daughters.This film deserves to become a classic because it's one of the most original and least predictable movies you'll ever see. You know it's probably going to work out alright in the end but how it gets there is both hilarious, thoughtful and a bit of an emotional roller coaster.
A lot of the movie's humor plays out like an exaggerated cartoon or perhaps stylized reality but it works really well because it feels like there's an element of truth in the base of the jokes.
Toni Collette's character (Shaz) could easily become the focus of several movies given her back story of weaving her way into people's lives. For me it's just awesome to see Toni play not just an Aussie but a broad, stereotypical Aussie (dare I say 'bogan') with a few character traits that clearly show she's actually not that stereotypical really.
It's difficult to say too much about the film without spoiling the unpredictable nature and surprises as everything unfolds. However I know I wasn't expecting giant shark stories, porcelain doll carnage, or even a couple of memorable cameos by Sophie Lee (Toni's arch nemesis in Muriel's Wedding).
My only (very minor) irritation about this movie was that it references American culture in a few places where, it might have been more likely for an Australian equivalent reference. Particularly where Barry Moochmore relates the story of how his father thought he and his brothers would grow up to be like the Kennedeys. It's not out of place but it feels like the reference is made so international audiences can know who he's talking about rather than have him reference someone only Aussies may know.
The film does contain coarse language but don't let that put you off. Everyone turns in a solid performance in what is a laugh out loud comedy where everybody is just a little bit mental to varying degrees.
Go and see it.