if you are wanting to stay at the Caravan Park -
very nice, and I would recommend it, like Bush camping but with 240 & showers & toilets
but
if you DO want to do free camping
Marrinup - old logging town north west of Dwelly - well, there is nothing left of the town, just nice flat areas people camp at
entrance off
North Spur road - off Del Park Rd - about 15km before the town on the right
or
Off Grey Street off Pinjarra - Williams road - about 10kms before the town on the left
then you can look at the Old POW camp thats just near by as well -
http://www.ozatwar.com/pow/campmarrinup.htm