'Step Up 3D' Chick Sharni Vinson -- Skin & Bones on the Beach

"Step Up 3D" star Sharni Vinson showed off her alarmingly slender bikini body on the beach in Sydney this weekend.

The 28-year-old -- who is currently dating "Twilight" muscleman Kellan Lutz -- could really use a cheeseburger about now.

So, we gotta ask ...