Hi Weta,
Good approach mate! There is a great little book called "priceless camping in WA". It lists most of the free or super cheap camping spots all over this great state. It costs about $28 and will save you that much coin in one nights accomodation at a free camp.
Quobba doesn't really offer much to my limited knowledge, but there is a great spot (which is not a secret spot) called Blowholes. As you approach the "T" intersection where the "King Waves Kill" sign is, turn left instead of right and drive to the campground. There is a beaut bay there with some waves (out the back) and flat water and nice sandy beach. Fun and safe kiting.
Camping fees are $5 per vehicle I think, which is payable to the ranger when he/she visits. There are long drop toilets and plenty of room for a tent.
I'll leave the locals to let you know about other spots south of Carnarvon. Have a great stop over!

KH