On the "Booking Summary" page where it asks you to "review you flights" if you scroll down it'll say "Your fare conditions". Open that up and have a look. It'll say something along the lines of:
Changes to your ticket
Time/date changes permitted at any time for the difference in fare. Changes subject to availability
Refunds
If you cancel a refund is permitted, subject to recalculation of the fare for any journey flown. There are no cancellation fees.
Service fees
To cancel your booking it is free online or A$40 by phone. To make changes to the date/time it is free online or A$40 by phone.