I agree, compared to the winter months in Pittsburgh the weather in Sydney, Melbourne, Perth is not "cold" but you could ht days where the highs are 50-60F.
Not really beach weather.
If your set on Sept look at going further North - Cairns, etc.
We spent a week in Broome at that time of year. Weather much warmer, beaches were empty, not much to do outside of the beaches (can do a pearl tour, salt water croc farm, camel tour, etc) but quiet at night although we did find a roadhouse with live music, lots of drinking, etc.