Just heard back from a Google Australia spokesperson regarding the Nexus One's launch in Australia. Here was the official response:
We do hope to allow people in other countries to purchase the Nexus One and future phones from Google soon, but I don't have a time frame to share.
Considering it's an unlocked phone, it will work on all the Australian mobile networks should you manage to pick one up from one of the four countries it's currently available in. However, you won't get the full NextG experience as it only supports the 2100 and 900 MHz UMTS spectrums - the same used by Voda, Three and Optus. It is Quad-band GSM though, so you'll be able to get 2G (and EDGE) service on Telstra should you choose to go down this path.