In some hotel programs, such as InterContinental/Holiday Inn, the AAA rate is often available; it typically matches the "advance purchase" discounted rate but it is cancelable. Most places it's cancelable by 18:00 or 16:00 day of arrival; others it's a day before.

If you don't have a AAA card, it's well worth it; I can say that I've never ever been asked for my card when checking in or renting a car.

But even if you have to pay the higher cancelable rate, sounds like with 4 travelers to SYD you have a non-trivial chance of not making it. (or having a mix of C and Y seats, or having half your group leave one day and the remainder the second day).

Also keep in mind that Australia requires an electronic visa called an ETA (electronic travel authority) for US travelers (and many other passports). You can do this on-line for AU$20 (about US$15). You can do it online here:
http://www.etaaustralia.com.au/en/ApplyForEta/index.php
and it's good for a full 12 months, multiple entries.
If you forget, or are lazy, United can do this for you (call reservations) or when you "check-in" ... but they'll charge you $25.