This image from one of the on-line Australian archives shows recently arrived M20s with War Department serial numbers at the rear. The machines in the foreground show that this number was roughly sanded off, an irregular black panel applied (with runs and sags) and then the Australian census number applied on top.

If you're looking for authenticity, then this is how it has to be done.

Much of the Australian records have survived so with some detective work, it should be possible to confirm WD number and Australian serial.