G'day Watchdog,
It's in our AIP, we use 'group form' for callsigns as you noted, five eighteen instead of 5 1 8 and pretty much individual digits for everything else except for full 'hundreds' or full 'thousands', except QNH which is always digits.
There are three or four pages on "transmission of numbers" and of course most of it doesn't apply outside the Oz FIR.
A dog with a watch eh?
Regards,
BH.
(Not ATC but a long time Oz pilot.)