ABOUT three-quarters of the way through “Somersault,” Joe, a good-looking farm boy, tells Heidi, the blond nymphet he has been sleeping with, that she has emotional problems and needs help.

He sounds off after catching the 16-year-old nude in her apartment with two guys she picked up at a bar.

When Joe asks why she brought them home, she answers honestly: “I didn’t want to be alone.”

The audience has known that Heidi needs help ever since the opening of the powerful Australian drama – when her mother catches Heidi in bed with the older woman’s tattooed, layabout boyfriend.

Heidi high-tails it out of town for a small ski resort, hoping to hook up with a man she had met earlier.

But he wants no part of the emotionally fragile teenager, who proceeds to throw herself at any man or boy who happens to cross her path.

One of them is Joe, who can’t decide if he loves her – she demands to know if he does – or is merely lusting for her slender body.

In fact, Joe can’t decide if he likes men or women better.

Heidi proclaims love for Joe, but it’s hard to know how she really feels, considering that she readily trades sex for affection.

“Somersault,” the feature debut of Aussie writer-director Cate Shortland, showcases a brilliantly realistic performance by Abbie Cornish as Heidi. She’s a provocative mix of naivete and ripe, unbridled sexuality.

Sam Worthington, as Joe, leads a potent supporting cast. But it is Cornish, who was 22 when she made the film, who amazes.

Could she, perhaps, be the next Nicole Kidman?

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SOMERSAULT

[*** 1/2] (Three and one-half stars)

Aussie nymphet.

Running time: 106 minutes. Not rated (sex, nudity, drugs, violence). At the Sunshine, the Empire and the Lincoln Plaza.