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What's your general fitness level like, and are you a strong swimmer?
If you are already confident in water then you are ok to practice on your own in safe places such as lakes, rivers as long as the weather is good, no currents and not windy.
If you are not a strong swimmer then best to join a group where others can assist you if needed.
As for specific areas I'm not sure, but I think the Nepean river could be an option.
Good luck and have fun!