You might find this helpful: http://en.wikipedia.org/wiki/Climate_of_Sydney So on average the coldest day of the year is 8 degrees Celsius (46.4 degrees Fahrenheit), and the coldest temperature ever recorded in Sydney is 2.1 degrees Celsius (35.8 degrees Fahrenheit). If you're just travelling down the coast, it won't get any colder than that. If you travel further south than Sydney, or you travel into the mountains, you might experience lower temperatures. Up north you'll need to be prepared for warm-ish temperatures of 20-30 degrees. But further south might need to dress warmly for temperatures as low as 10 degrees (maybe slightly colder if you're unlucky). The weather will definitely be a little warmer at the end of August, so you'll probably miss the worst of the cold weather.

Note that Darwin and Sydney have different climates. In Darwin you can get away with t-shirts all year around and August will be quite warm, but Sydney is cooler although still not as cold as northern Europe. August should be late teens most days so you only really need a light coat for the nights.