If you try the one month membership, the currently available networks are Virgin, Vodafone and Telstra.
My payment page is showing that it is cheaper to purchase one month at the moment. My one month price shows AUD 14.00 AUD 17.00. Does your one month price show the same?
This thread was created nearly a year ago and Optus was the only available network for Oz Runescape purchases back then. It seems to have changed quite a bit since. sms payments have always been a very changeable thing.