It is possible to camp for free in your car or van at the Dee Why Beach. The beach including a large parking area is located in the north of Manly Beach. To drive the downtown Sydney, it is about a 30 minutes drive (20km), Manly Beach is only a 15 minutes drive away (less than 7 kilometers). This beach is not as known as Bondi or Manly, yet it is very beautiful. Just like Bondi it has a natural pool and many bars and restaurants around.
If you park there between 7am and 7pm are required to pay a parking fee of 8 dollars. If you park here after 7pm there is no fee to pay. However, you can also park for free in the streets if you are lucky enough to find a parking spot. Make sure to check the street signs for any restrictions.
There are showers and toilets available, which is great. When I was there I did not find any BBQ facilities, but maybe you have better luck, since the park area is quite large. In addition, you wake up right by the beach and that is pretty much priceless, right?