hardly an Australian saying...

The quality of mercy is not strain'd, it droppeth as the gentle rain from heaven upon the place beneath: it is twice blest; it blesseth him that gives and him that takes ... though justice be thy plea, consider this, that, in the course of justice, none of us should see salvation: we do pray for mercy; and that same prayer doth teach us all to render the deeds of mercy. I have spoke thus much to mitigate the justice of thy plea; which if thou follow, this strict court of Venice must needs give sentence 'gainst the merchant there. [Portia, to Shylock in "The Merchant of Venice"]

the rest is just a paronomasia, a kind of shaggy-dog, a feghoot.