there is simply no need to hold an ATPL before having a command on a type that requires it.
It is nothing to do with being a captain. It is because there are airline operators (Virgin is just one) and regionals in Australia that require the ATPL before a candidate is even considered for interview.
So you can have 1500 hours in command of Cessna 150 as a flying instructor and providing you have a current instrument rating and a twin endorsement it will get you an interview. The 50% of co-pilot time is the problem especially as that rule is peculiar only to Australia