Joe, my anecdotal experiences may not be the best predictor, but here's been my experiences on two trips that time of year --
In mid-October '15, I observed at the Markdale site, along with John Hoey and Kemer Thomson. I think the forecast was looking pretty bleak the day I flew to Sydney, so I was surprised the first half of the first night was clear (perhaps 4 hours). The next day (Sunday) it rained all night, so not a great start at this point. But then we had 5 consecutive all-nighters with very little or cloud-free conditions. So, I ended the week exhausted from the number of hours of observing (6 out of 7 nights). I check my notes again and SQM readings were consistently above 21.8 with several readings between 21.9-22.0. I believe the star party last October/November had similar luck with the weather. Locals Dennis Beckley and Bob Douglas attended, though you probably would have to contact them directly for their experiences.
In early November 2010 I observed at Coonabarabran, which is further from Sydney (perhaps a 6 hour drive). The weather prospects looked dismal the few weeks leading up to the star party with consistently cloudy conditions and rain throughout New South Wales. But although there were clouds during the daytime, it cleared by sunset on the first 4 days of the star party and I got in quite a bit of observing under impressive skies (SQM readings generally 21.85-21.89). But we had clouds/rain for the last 3 nights, so we ended up with 4 out of 7 clear nights.
So, yeah, there is a risk with clouds/rains but I've rarely (if ever) returned from a trip to Australia bummed out by the weather (say with over half the observing time missed due to weather). Conditions are drier further in the interior or perhaps on the west coast, but then you're really on your own in terms of planning a trip, and would have to lug along your own equipment.
Also, if you haven't been to southern hemisphere before and are only planning a single trip, then I would recommend going to the March/April star party as you get the full Milky Way overhead experience with lots of great nebulae/clusters that you haven't seen before.