Here you go, Rob:
From http://www.climb.co.nz/Places/Australia/MtArapiles/Mt-Arapiles-Rock-Climbing.htm:
Seasons
Spring (September to November) is generally fine, however it can rain with little warning.
Summer (December to February) is dry and hot, with climbing often restricted to the shady gullies
Autumn (March to May) is generally fine with cooler nights
Winter (June to August) while it can be wet and cooler, you can get some near perfect days.
I've not climbed there, but my understanding is that when it's too hot at Arap, Mt. Buffalo or Tassie are good warm-weather alternatives.
You may want to check out this site, also-- great for local beta/partners. http://www.chockstone.org/ (I'm on there under my Aussie nomer, "Shez")
I'll be heading Down Under next year to catch up with some old friends there, and to climb(of course). If your trip pans out, maybe we could rendevous.