It depends on what kind of paint you are using. Water and Windex work great on acrylics, but will do nothing for enamel or laquer.
For cleaning any kind of paint use laquer thinner. This will eliminate laquer, enamel, or acrylic.
For enamel or acrylic, mineral spirits will also work but it won't cut laquer.
For acrylic a mixture of 2 parts distilled water, 1 part Windex, 1 part Simple Green works well but I don't know if you have those available in Oz. Any ammonia-based window cleaner will work well but can cause the finish of your airbrush to change darken. Rinse it thoroughly in water afterwards.
Edit ... That's what I get for not reading the post thoroughly. I thought he meant for "Cleaning" his airbrush.
Anyway, most of what I wrote still applies, but for thinning acrylics for spraying I normally use isopropyl alcohol. You will sometimes get better performance using the recommended thinners. Specifically, using isopropyl alcohol with Tamiya acrylics will always cause them to dry flat even if the paint was supposed to be glossy. Additionally, some acrylic thinners (specifically Tamiya again) contain some reterder to slow the drying time down. This helps with the buildup of paint on the tip of the airbrush and also allows the paint more time to level out as it dries. Acrylic retarders are available at most art supply stores, and I usually add a couple of drops to my paint cup when spraying acrylics.