From the Jetstar Terms and Conditions (
http://www.jetstar.com/au/en/conditions-of-carriage )
Originally Posted by JQ
9. SCHEDULES, LATE OR CANCELLED FLIGHTS
9.1 Schedules
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(c) If, after you pay for your Booking, we make a significant change to the scheduled departure time of your flight or the flight is cancelled (except where this is due to circumstances beyond our control) and:
this change means you are unable to use your Booking for its intended purpose, and
we or our Authorised Agents cannot book you on another flight which you are prepared to accept;
we will give you a fare refund. Unless provided for in a Convention or any applicable law, we will not be responsible for paying any costs or expenses you may incur as a result of the changed time or cancellation.
I would suggest that a 7 hour time change is definitely significant and that you should be able to cancel without penalty based on the T&Cs unless JQ can rebook you onto a flight at a similar time.