Taking off my preferred AvGeek hat to put my lawyer hat back on for the last time this week. As has been outlined, it's pretty clear what Qantas has chosen to do. It's all legal (so far). Transparent and in good faith? No.
1) Qantas is racing against the clock to push their marketing blitz HARD to lure people into accepting the "offer" to cancel their flights for a voucher. Their own service line when you call 131313 uses wording like, "you must take action by 31 March" to scare people into pulling the trigger thinking they'll lose out if they don't. That's not true, nor is Qantas ever saying it is. By doing so, however, you are technically initiating the change and cancellation, and therefore not entitled to a refund, and never will be, even after the fact.
2) Qantas knows these flights won't operate (and have even said so publicly and stopped selling them), but hasn't officially cancelled them in people's reservations. They'll get away with not doing that for a few days on the basis that it takes time to manage and process, but eventually, they'll have to or risk some major legal risks. They won't push their luck, but they'll get as close as they can to the edge. When they do cancel, and they will, despite #1, now you're entitled to that full refund.
3) Once Qantas cancels your flight, it does NOT matter whether the cause of cancellation by Qantas is within or outside Qantas' control for the purposes of a simple refund. Nor do the fare rules have any relevance. Those rules apply only to your decision to cancel, not theirs.
4) Qantas may still push vouchers once they initiate a cancellation, but you are entitled to insist on a refund and should refer them to their own Conditions of Carriage. It pays to be aware, and if you don't ask for it, they can say it's because you simply didn't want it and "agreed" to an alternative. Cheeky perhaps, but hey, we all clicked the box to claim we read those terms no one reads.
I suspect what's most likely to happen is that Qantas will push vouchers and not cancel flights for as long as they can reasonably get away with it,. Once they do, they will then contact passengers and offer or even automatically convert the ticket into the voucher with no proactive mention of the option to refund. I also suspect, however, that if challenged by an informed passenger, they'll cave quite quickly and give you the full refund.
Because if they don't...
Airlines are always a fun source of community complaint and Qantas does not want a news story on 60 Minutes or A Current Affair exposing them for unequivocally denying to honour the same terms and conditions they're happy to throw in your face when the issue is in their favour. Nor do they want a class action lawsuit, which would be pretty easy to get going.
So, take a breath and bide your time. There's nothing convoluted about it; all that matters here is who's doing what and when. Force them to be the one to take action. They will have to. In the meantime, unlike, for example, Air NZ, who's being upfront about what passengers are entitled to once the airline cancels the flight, Qantas has chosen to keep hush. What they've yet to do however is actually deny honouring their own conditions of carriage and until they cancel these flights, we won't have those data points.
TL / DR - Patience will prevail and the early bird won't get the worm this time. I get why they're employing this strategy (and people's memories are never as long as they claim they are), but I personally won't be caught by it. If you want a refund, do not press "cancel." Do not accept if you call and they say they can't because the flight hasn't been formally cancelled. Wait.