The path was in the region of 3200 km which is way beyond the usual 2300km or so the one-hop distance from Sporadic-E. It seems likely that on this occasion, the most likely propagation mode was double-hop Sporadic-E which is very rare at 144 MHz.
It looks as if just one WSPR transmission from each of the VK2 stations was decoded at 10:16 and 10:36 UTC. This was about an hour after sunset in Sydney which is 11 hours ahead of UTC.
It’s also worth pointing out that the reports from the WSPRnet website say that the VK2 stations were running just 10 watts.
Tropo?… As the path crosses the Great Australian Bight, it’s always worth checking to see if that was a factor.
The forecast was for some weak tropo across the Bight but nothing special and it doesn’t seem to extend inland.
VK6CPU in Perth was also heard by VK5AYD in Adelaide at around the same time over a distance of 2149 km which was likely to be via Sporadic-E.
Timestamp UTC Call MHz SNR Drift Grid Pwr Reporter RGrid km az Mode
2020-12-15 10:56 VK6CPU 144.490510 -24 3 OF78wb 5 VK5AYD PF97ja 2149 99 2
It would seem as if double-hop Sporadic-E was the most likely mode of propagation.
Although though this was a one-way reception report, it does suggest that a two way 3000 km+ contacts across Australia should be possible in the future.