Ugh - the degree to which Labor doesn't deal with postals drives me insane.
Keep in mind that because Australia has compulsory voting the turnout is typically over 90% so its all a zero sum game. Sure Labour good make a much bigger effort to get their identified voters to vote postal...but for the most part it would just subtract from the number of people voting Labour pre-poll or on election day - so it really doesnt get you anywhere.
It would only make sense for Labour to make a much bigger push for postal votes if they genuinely belived that there was some hitherto unknown pool of non-voters who could be motivated to vote Labour by post and who would not otherwise vote any other way.