See below for cheat for non-Optus customers.
$20 Gold Class tickets for Optus customers
$20 includes the booking fee too.
Not valid after 5pm Saturdays and Public Holidays.
Cheap luxury :)
Expires October 20 and can't be used for presale (eg twilight).
The following workaround seems to be changed and doesn't work for now. Optus customers can still use the link above.
Thanks to sonicentropy and tko6070 for the cheat way around:
Change end of the URL &bookingsource=web to &source=optus