From my own experience at Dwellingup Gnanga capsites, unless you get there BEFORE the public holiday, you've got buckleys chances of an UNBOOKED site.
You can book sites there, especially at the new (OLD Baden Powell site ) Stringers, Chudich are all booking sites, but like they say, yu gotta get in early, by that I mean REAL EARLY, like 6 months early.
However, just for the record, We avoid going up there during public holiday weekends, far to much mayhem and people for us.