On
ebay at present, there is a used copy of this stamp for sale via Ian Perry (seller ID pezbluenose). They are very scarce.
The 1999 edition of The Australian Commonwealth Specialsts' Catalogue (ACSC) - Section 10 - Postage Dues states that used copies exist with two separate cancels, viz:
"This variety is rare and may have comprised only several sheets. One mint block of 4 existed, but has since been broken up*. Used examples are recorded dated February 1921 at the Sydney Delivery Room (this seems to reference Ian Perry's copy), and 9 June 1922, cancelled in New South Wales."
* Millennium Auctions sale #11 21 April, 2004 for lot #468 which realised $10,000 in that sale:
1913-21 Thin Paper 3d Carmine and Pale Green, WATERMARK SIDEWAYS, BW D100a, SG D82a. The 3d Wmk Sideways is one of the great Postage Due rarities of the world, with less than 12 examples known to exist mint or used. This stamp is believed to be the only surviving unmounted example, deriving from the block of four that was tragically split up more than a decade ago. A superb stamp
A little research today reveals four used copies.

This is the stamp for sale by Ian Perry, the clear FE 21 date indicates this is the former stamp referred to in the ACSC.
Now, this is where it gets interesting. The ACSC does not refer to the number of stamps known, but does refer to two separate dates.
Here is a stamp clearly dated 9 JU 1922 but with only NSW visible in the cancel, it meets the description of the second group of dates in the ACSC:

This stamp was also offered at the Millennium sale noted above.
However, I have also found these two images:
This from Grosvenor Auctions sale #72, 4 Oct 2012

This from Stanley Gibbons (Australia) Melbourne auction 25 March, 1992.

Notice how the Grosvenor shows "N" at the top of the CDS and the Stanley Gibbons one shows "NEW".
I suspect the Gibbons one has not been taken into account by the ACSC as it clearly shows that the cancel would be Newcastle.
It is no coincidence that the last three are all clearly dated 9 JU 1922.
Here we have two clear groups of cancels, 16 months apart. It goes to suggest that there were two separate sheets of 120 issued. I work on a 5% survival rate. 8 are stated to be known, there should be another 4 out there (at least).
A note here for anyone trying to research stuff. Google images is a great help. I used both the SG cat ref of D82A and the ACSC cat ref of D100A in Google images to find these stamps (the black-and-white image is from my catalogue library). This leads to the web pages with the information.