I've seen the idea of an Interwar Populate or Perish sheme floating around here whenever someone brings up making Australia stronger, but I can't figure out how to have this happen myself so I figured I'd make it a challenge/ask for help on here.
The reason that PoP happened after WW2 was fear of Japan invading made them think that Aus think its population was too small to defend itself, so you'd probably need this fear to manifest in the Great War instead of its sequel - but how?