JetStar cancelled a flight from Sydney to Melbourne on December 6 2019. Ok, your plane has technical problem. It's happened, even in normal companies.
Ok, you do not want to spend your money and would like to offer a flight the next day to try another time. Of course, flights between Sydney and Melbourne are such infrequent, and I can wait for one-two days. You said that no tickets available for other companies... Do you really believe this information cannot be checked immediately?
In case of a specific nomination for the most inefficient and customer-unfriendly helpdesk - you have a wonderful chance to win.
Finally, the attempt to fly with you cost me additional S2500 for three tickets. See you the first and last time.
Domestic
Australia-Sydney
Australia-Melbourne - Avalon