I live in a place called Wollongong which is 1hr south of Sydney. September is a great time to visit. The weather is warming up, people are starting head to the beach and the water temperature is'nt too bad.
You can definitely get away with wearing a t-shirt during Septmeber. Average temperature is 75 - 80.
As for beaches you can head to places such as Bondi or Manly. I recommend you head down to Wollongong as it has endless coastlines and you won't get the crowded beaches as they are in Sydney.
If you need more advice or info, i'm more than happy to help.