Qantas seems to have clarified the issue a bit more now. From their Facebook page
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If you’re booked to fly up until 31 May 2020 and no longer wish to travel, you can avoid change fees by having the value of your fight transferred to a flight credit. This credit can be redeemed up until 30 September 2020 or 12 months from the original date of ticket issue, whichever date is later.
For anyone who’s already made use of this offer and received a credit that expires before 30 September, we’ll extend it to 30 September 2020. It can be used for any travel dates available when you go to redeem it.
Go to Manage Booking
Manage booking | Qantas AU on Qantas.com, click ‘Cancel’, then ‘Voucher’. Your flight credit may take some time to process, so please be patient.
Our Call Centres are under significant pressure responding to the rapidly evolving travel situation. Our call centre and social media teams are working around the clock to proactively contact and re-accommodate our customers. At this stage we are prioritising those with imminent travel. We ask that you reconsider your need to contact us unless your travel is commencing within the next 72 hours.
If your flight was booked through a travel agency or third-party website, contact them directly.
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The way I read it, everyone’s voucher will be up to 30 September 2020 if they booked before 1 Oct 2019, or otherwise it will be later. That voucher will need to be used by the due date, but the flights won’t have to be taken, so you probably have up until at least August 2021 to travel.