Nope, you'e thinking of the Dog Fence.
The Rabbit Fence (pictured in my earlier post above) divides the more-or-less fertile south-west corner of Western Australia from the arid interior. It's maybe 2000 or 3000 km long. (And yes, its function these days id to keep Dingos and Emus out, but it was built in a vain attempt to block off the rabbit.)
The Dog Fence, at 5400 km, is the longest man-made structure on earth. It runs from near Ceduna in South Australia to near Brisbane, and is still maintained. (So, for that matter, is the Rabbit Fence, though I'm not sure if anyone knows quite why anymore.)