Now he wants $AUD5,950 for this dodgy fake cancel example.

Geoff Kellow in the 2020 3rd edition of the Australian Commonwealth Specialists' catalogue, Postage Dues including New South Wales and Victoria, BW Publishing, Sydney pp10/60 states:
"At present, two examples of the 2s, 10s and 20s*, all cancelled with Parcels Branch Sydney datestamp in violet are recorded, and represent the only acceptable used stamps".
He offers a Brandon certificate but my own experience is that they are worthless for these issues.
I'd trust the researcher over the dealer every time.
*a third example of the 20s has surfaced subsequent to the publishing of this catalogue.