The location of Wentworth Park is located very centrally in Sydney where you can camp overnight. You can even walk to the Sydney CBD in half an hour. To reach the Sydney Opera House you need about 50 minutes by foot. At the park you can even find public toilets, which always comes in handy 🙂
Be reminded that the car needs to be moved every 24 hours. And they do might enforce that. But that still gives you plenty of time to explore the city and even for a good night sleep. Check if your tyres have been marked with chalk – if so it means that your car is being checked if its being moved within 24 hours.