Im not sure whether its an actual treaty or whatever, but there was the Calwell's Mass Migration program after WW2 to boost numbers due to the whole 'Populate or Perish' idealogy. The issue of migration was a bipartian issue until the 1990's with John Howard.. The government was still strongly on the WAP (White Australia Policy) so they promoted the intake of Europeans until 1972 when Al Grasby brought in the Multiculturalism Policy and the diversity was welcomed.