I already received an updated booking confirmation and next steps email a few hours ago.
The email states:
"Any future date changes are subject to our standard fare rules, and any applicable fare difference.
If you are proceeding with this offer there is no action required from you.
If you do not wish to proceed with this offer, you can request a full refund by replying to this email directly confirming that you wish to cancel and refund this booking. We will then process a refund to the original form of payment. Most refunds are processed within 7 days, however can take up to 4 weeks, depending on your financial institution."
Does not mention when I have to decide by - maybe qantas are keeping it flexible as they would rather people cancel these tickets.