Generally, if you want to swim, you head north in their winter (ie North Queensland) although you can swim in southern Queensland esp Sunshine Coast and Gold Coast if you’re okay with chillier water. In their summer, beaches from southern Queensland downwards are good. If travelling in their school holidays, beach areas are super popular and you need to book well in advance.
I like the Noosa area in southern Queensland. A more chichi beach area with lowset development and lots of nature, great food and somewhat of a more linen-clad set. Even in winter, you walk along the path through the national park and you see scores of wetsuit-clad surfers along the gorgeous beaches then realise how many are 50yo-plus women and men just living their best lives. Inspiring. If you are travelling solo, lot of organised day trips you can join, eg kayaking, 4WD trips along deserted beaches, etc. I think you can fly there direct from Sydney.
Sydney has spectacular beaches and has the benefit of being able to enjoy both urban and natural settings at the same time. For example, you can get the ferry to Manly and enjoy the lively beach scene, good food, etc and then do the great 10km Manly to Spit walk. It’s one of my favourites. There really is no excuse for not having amazing coffee and food esp if you do a little research.
https://www.sydney.com/things-to-do/nature-and-parks/walks/spit-bridge-to-manly-walk
Other things to do if you’re travelling solo in Sydney might include a cooking course at the famous Sydney fish market, the Harbour Bridge climb and checking out the rock pools which are a pretty unique Sydney feature.
https://www.sydneyfishmarket.com.au/Sydney-Seafood-School/Cooking-Experiences/Book-an-Experience
https://www.sydney.com/things-to-do/tours/bridgeclimb-sydney
https://www.russh.com/sydneys-best-rock-pools/