The travel.com.au site now lists the following rule restricting number of intra-Australian transcons:

WITHIN AUSTRALIA - ONLY O N E NONSTOP/SINGLE
PLANE SERVICE IS PERMITTED BETWEEN THE FOLLOWING
POINTS:
BNE/CNS/SYD-PER;
MEL/SYD-DRW.
EXCEPTION: NO RESTRICTION BETWEEN SYD-PER FOR
PASSENGERS ORIGINATIING IN PER WHEN IN CONJUNCTION
WITH TRAVEL TO/FROM AFRICA.

The "Originating in Perth" has been added. It used to say "No restriction between SYD-PER when in conjunction with travel to/from Africa" I don't know if this rule applies only to Oz-issued tickets or not.