I asked Ursula hall (who offered me yesterday) if the registration fee thing would be refunded if I decided to go to Monash instead. got this reply:
"Yes, they can be refunded in full. However we are less likely to do so the closer you get to the contract start date of the 9th Feb, as it gets harder to find someone to replace your spot.
Given the timeframe, you will have to make a decision before second round offers go out, or risk losing your acceptance fee. I realise this makes your decision very difficult, but keep in mind that waiting prevents another student with a first round offer from getting an offer of accommodation."
I'm probably gonna end up declining though, and know a few people who will, so it seems like there are plenty of spots open to new undergrads because of the whole accommodation guarantee.