My understanding from the Royal National Park is that if you're driving straight through from Loftus to Otford via Audley and don't stop you don't need to buy a national park pass, however in that case the road is considered a public road excluded from the park. I thought the same applied to all other parks, so driving through Riverside Drive here without stopping doesn't need a pass, but I'm not sure... The national parks website seems to indicate that if your vehicle enters the park you need a pass.