Why Did Canada And Wales Mint Australian Coins In 1981?

I think we are all reading too hard into it.

I personally believe it's got to do with the metal price. At first I thought there's something to do with seigniorage but that didn't make sense as producing 50 cents would be more profitable.

I looked at this from a different perspective and remembered this was the era from the Hunt brothers where they tried to corner the silver market. Various metal prices rose in 1979 and collapsed later in 1980.

I reckon this would also explain why mintage dropped in 1979. It slowly increased the next year and perhaps when metal prices were too good to be true, RAM made the decision to mint the more expensive small change cheaply even at the expense of contracting to other countries. Guess it's like hedging in some way.