We had a great holiday in Western Australia last December. Originally we wanted to head north, but to everyone's advice they told us to head south, and we loved it. From what we were advised, the weather gets very warm to the north.
We spent a couple of days in Perth, drove to Albany for a few days, then to Margaret River (with a truffle lunch in Manjump), then a few days at Bunker Bay before returning to Perth. Great weather, awesome wine and food, and a wonderful holiday.