If the WT were honest, they'd set up some sort of pay as you go (PAYG)* scheme to allow the COs/missionaries to have tax and superannuation** contributions deducted. It would make life easier for all concerned. However, if Sydney Bethel is as honest and ethical as I believe them to be, I imagine that they encourage the COs etc. to treat those donations as "under the table" payments.
If anyone knows differently, or has some knowledge of how this sort of thing is handled here in Australia, please let us know. I always find this sort of thing interesting 💭 🤔
* Basically a withholding tax system
** Something like the American 401k thing, but it's mandatory for all Australian employees to have an account with a superannuation fund.