Yes, that is only a proxy for the overall all infection fatality rate. However, given the contact tracing, testing, and medical systems have been quite robust in Australia, it shouldn't be too far off the mark.
Furthermore, the 3% figure only applies to the population as a whole. According to the ABS, CFR climbs to over 30% for people 80 and over. Similarly, a recent American study found it to be about 21% [1].
So, back to the original point, it should be very easy to find a mortality signal in a RCT, which included that cohort.
[1] https://www.medrxiv.org/content/10.1101/2021.04.09.21255193v...
