On the banks of the Hawkesbury river and close to Sydney. Not free but value for money.
Overnight camping fees can be paid online at NPWS, and park entry fees are to be be paid at the gate - bring $8 in coins. Gate locked overnight but when booking online you are given the code for the gate if you need to leave.
Showers, toilets, a few shelters, and wood BBQ's but bring your own wood. Quite popular, so get there early. Clean and neat areas, and Rangers patrol often.