@bigbopper, actually when it comes to ash cloud forecasts they're pretty good. It's an international collaboration, with a type of 3d infra-red (from satellites) layering and computer modelling. A safety buffer is built in. A lot has been learned about this, fast, over the past decade or so.
Looks to me that the cloud on the Darwin VAAC site is likely to be far enough South that it's unlikely to affect your flight, anyway. You may notice (depending on where you are, and where the upper ash cloud is) that the flight levels off initially a bit lower than normal for a while, before continuing climb.
The NZ Met service gets it pretty right, most of the time, in regard to weather generally, in my experience. A few noticeable "misses", but the important stuff is usually good.