I have to be honest and admit I've not heard of any crop circles being found in Australia. Except for the ones created by wallabies high on opiates (and no i'm not talking about the Australian rugby team).
With a quick search on the internet I came across many mentions of "The Tully Saucer Nest" of 1966. According to ufos.about.com describes this as a "very compelling case of physical evidence".
According to reports a man called Pedley saw a very large object rise from a swamp up to 30 feet in the air. Once the object was gone Pedley went to where he'd seen the object and found an area of "mashed down vegetation" with "water circling around the area as if recently disturbed".
Later Pedley went back and noticed there was a "floating mass of reeds, which were in a clockwise pattern". It was 30 feet in diameter. Pedley reported this to the land owner who then remembered his dogs had been acting strangely earlier that day.
Pedley and a work colleague decided to photograph the anomoly and found they could "swim underneath the mass, and that the lagoon was smooth underneath it".
Later Pedley made a police report who then reported it to the RAAF. Soon the swamp was full of reporters and investigators. Many theories for the anomaly included helicopters, birds, whirlwinds etc.
Soon after, the farm was searched and 5 other "nests" were discovered.
According to the RAAF, they tested the mass and nothing anomalous or otherworldly was discovered. Their guess was that it was caused by a "wily-wily" or a dust devil which are common in that area. However it doesn't explain what Mr Pedley described seeing.