Not an Australian but I bet good money it is the exact same reason as everywhere else in the west: people have no time, no money, no social services, and really bad future perspectives with the ecosystem going to shit and capitalism running the world into the ground.
People need time to raise kids, people need money to afford kids, kids need education and attention. Provide those things, and provide a not-bleak perspective for the future, and people will be happy to have kids again.