You could pay dearly for a 650C bike for her, but that would not be worth it. There really is no simple solution, so it comes down to how handy you are, and your finances. You could get her a 24", and put some slicks on it, as suggested. You could get her a small MTB with 26" wheels, and again, put some slicks on it, as well as cut the bars down a bit, and install a shorter stem. Without knowing how big she is, it's tough to say.
For a child and a commuter, the frame weight is not a huge issue. I've done 40k on an 18kg single speed BMX bike as a kid. If you can find a mountain bike frame that fits her, get a nice 26" commuter wheelset built for it, and add some racks. You can't lose with items that can be moved from bike to bike. And if Melbourne roads are anything like Newcastle roads, you'll want a flat bar to give you a bit more upright position anyway, so a MTB makes a good choice again. Spares are also more plentiful, and cheaper.