AFAIK, there is no mechanism for collection of price differences as a result of fares dropping. It is a gamble -- you loose if the fare drops after you buy it, but you win if you get in before it rises again. In any case, the N class (red-e's) are pretty good value anyway. Just get in fast if you see a bargain fare - they arn't there for long.
Hope you enjoy the trip in Oz.