Yes,
Pukka, it is viable. I usually do it each quarter.
I posted on it a while ago in this thread, see post 922:
http://www.australianfrequentflyer.c...tml#post605051 (ATO (tax office) payments by credit card)
CBA changed the terms a while ago so you can only have
one Great Rate-d transaction on a card at any time (and 6 maximum per year). If you are paying tax quarterly, it basically means you can only Great Rate two tax quarterly payments per year, if you want each Great Rate-d transaction to last the full 3 months before you pay it off.
The ATO's 50k limit is unhelpful, if you have larger tax instalments due, because you can only Great Rate one 50k tax instalment on a card at a time.
Still, it is worth doing. I now run two Diamond cards to do this, alternating each quarter which card I pay the tax on and Great Rate. That way I can use the full 3 months. Paying the extra annual credit card fee is worth it to me to be able to do this.
NC