Climbers are allowed to use outdooor areas just like any other user group, as pointed out before. Just like any user group they will disturb the peace, make a bit of noise and break the odd twig.
However being more sensible than most groups, organisers could start the comp by a briefing explaining where to pee, what to do with rubbish and where stay clear of. At the end of comp they could organise a “Treasure Hunt”, which is a euphemistic term for everyone picking up rubbish (any old rubbish, not just climbers, so the site at the end is actually cleaner than originally). Hence people get to use and appreciate the site.
As for the Aboriginal Site issue, check it out if you have not done so, the register of such sites (by grid coordinates, so you will need a map) is available on the web. Perth and WA have hundreds of such sites, most of them have no resulting restrictions on access whatever (Wallcliffe and Llama Rock being 2 examples). The ones that do tend to have large signs to that effect.