Well, that didn't work out. As far as SAS were concerned, they have just moved us to the later flight and no full refund is possible.
Australians are barred from leaving the country anyway because of covid-19, and I suspect SAS is aware of this restriction. It's possible to take legal action against the Australian subsidiary under consumer rights legislation but (a) it takes a lot of effort, (b) can be expensive, and (c) there's no guarantee of success given the new world we now live in.
It is best to cancel and take the loss.
At least Finnair were very helpful for our MEL-OSL flights. Just waiting for the refund (which will take quite a while). These were business D class which I think are fully refundable.