Ok legally, from what I can see on the net you should be safe but I am not a lawyer and have not studied Australian nor international copyright laws. You also in this case have the plausible argument that although it is a fictional place the streete mappe of ankh morpork was classed as non fiction, at least in th uk. To be on the safe side try and get someone from the Australian publishers to give you a response, you never know they might even be prepared to sponsor your game!
If you can fill the unforgiving minute,
With sixty seconds worth of distance run,
Yours is the Earth and everything that's in it,
And - which is more - you'll be a man my son.