If the purpose was to encourage everyone equally to procreate, the benefit should be equal to all. Sure, some people 'need' it less, but that's irrelevant. It already is skewed - $5k to me is worth less than it is to a single mum who doesn't work, for example.
Australia's taxation system is progressive, so the more you earn, you pay more proportionately, so any benefit that you might get for higher earners should be less. Benefits shouldn't be a fixed % of income for all.
However, given finite government revenues (although with the billions in surplus it doesn't seem that!), I believe the government can better use this money elsehwere.