The 1966 20c coin was minted at the Royal Australian Mint in Canberra (28,196,000 coins) and the London branch of the Royal Mint (30,000,000 coins). I find this coin a little tougher to pick between the two mints. The water swirl that ends in a point next to the platypus’ head on the reverse is supposed to have no gap (or a very small gap) between the swirl and the head on the London coin, and show a distinct gap on the Canberra minted coin. However if the strike is slightly weaker on the London minted coins then there can be a small gap present and the coin must be examined closely to be sure of the mint of origin.
You can find out about the other 1966 decimal coin mint marks by looking at our 1966 Australian decimal coin mint mark article.