Can a single phase appliance (Australian 230 V) be wired directly to a three phase socket using a single active plus the neutral wire? If not why?
Typically yes, you can run a single phase load from a three-phase service.
An issue would be if it is a line-to-line load or a line-to-neutral load, and whether a neutral is present. Another thing is to make sure the voltage is correct for the application, and whether a high-leg is present (not sure if this applies in your area).