Depending on your mindset, that's either a really neat idea or a shortcut to madness, but either way the path to get there is pretty simple. Install the software and record and customise a series of prompts, then connect your compatible GPS (there's a list on the Garmin site, but most nuvi models except the 300 and 600 are covered) to copy them across.
During setup, there's no specific option for Australian English, so you'll probably want to choose the British English setting unless you're planning on impersonating Homer Simpson. Garmin estimates the full process takes about 30 minutes. One possible option is to get a loved one to record the messages, especially if their voice is more mellifluous than your own. Garmin Voice Studio is a free download for Windows only.