I'll simply say that having redeemed dozens of awards in economy, business, and first class on UA, I feel confident that I know how to look up award availability and it is there right now in economy class.

Perhaps there will be thousands of people trying to redeem awards to SYD and MEL, but I don't think so. And as someone who redeems 6 to 10 airline awards a year I think that if a person really wants to go to Australia with their free UA ticket that they have a good chance of getting an award if they give it a full effort and keep trying.

Personally, I am not that concerned if there is economy availability or not as I plan to be flying on a UA or Qantas First Class award to Australia this summer and I am confident about getting that also.