You've got to remember, Australia has now had decimal currency for a longer period of time than it had predecimal currency. Those 1966 coins have been out there in circulation for over 50 years. As such, the original numbers of 1966 coins, although huge to begin with, have gradually diminished, and the proportion of the coinage that bears the 1966 date falls each year as more and more coins are made.
I think the "wavies" are still quite obscure, as far as the "average person in the street" is concerned; as such, they would survive longer in circulation than a "scarce date" (like the 1930 penny) would have.
In other words, I think they're becoming harder to find simply because 1966 20 cent coins are becoming harder to find, not necessarily because people are noodling out all the wavies.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis