I have an older ACSC King George VI catalog which has a section on the 4d Koala. The really big value is in the Plate Number Blocks but there are some other plate varieties of interest (and value).

I've added some red circles to try and help identify. The numbers of the pictures identified with 198 are the C of A Watermark, Perf 14.75 x 14. The numbers with 199 are the unwatermarked version.
All of the pictures are hard to see, so you need to read the description and carefully compare to other copies to really understand.
d: Re-entry below base frame
e: Roller shift to "POSTAGE", "4d" and base frame
f: Color spot below base frame right of "E" of "POSTAGE"
g: Retouch to four lines above right of "IA"
h: Color spot below top border right of last "A" of "AUSTRALIA"
ha: Retouch to five lines above right of "IA"
i: Color in last white border segment at top right
j: Color flaw below left of first "A" of "AUSTRALIA"
ja: Retouch to four lines below first "A" of "AUSTRALIA"
k: Crack through top of last "A" of "AUSTRALIA"
I don't have the latest values, plus it is in Australian Dollars, but any of these items are worth 50x the value of the plain stamp. Types g, j, ja, and k appear to be worth about 200x.
Happy hunting!