There are plenty of methods mentioned in this thread. The basics, repeating earlier posts are to buy the Only1’s with discounted wish cards bought via CashRewards or EntertainmentBook using Amex Platinum Edge for 3ppd etc....for most the cost of the $100 Only1 comes out at $100.65, eg a 65c premium.
In your case your paying a premium of nearly $4 for every $100 spent. Unless your getting an unexplained benefit, your better off paying the ATO bill direct with your Amex and copping the $1.45 Amex fee per $100, but likely still getting half a point per $? Even cheaper if you use Visa/MC etc. In essence unless your getting the Only1s cheap AND getting 3ppd it’s not worth buying them.... IMO.