1981 20c struck on Scalloped Planchet
In 1981, part of Australia's 20c mintage was struck by the Royal Mint, Llantrisant. Among the 50,000,000 20c pieces struck in Wales, a small number were struck on scalloped edged planchets.
In 1981 the Royal Mint, Llanstrisant was also striking coins for Hong Kong, including the Hong Kong $2 coin, which has a scalloped edge. No doubt a small number of Hong Kong $2 scalloped edged planchets were accidentally mixed in with Australian 20c planchets, leading to the striking of a small number of wrong planchet Australian 20c pieces. It is unknown how many were struck but a handful or more were released into circulation and were initially discovered in late 1982 (https://www.downies.com/news/treasures-in-your-change-20c-what-to-look-for/).
1981 Australian 20c pieces with scalloped edges come up for sale rarely: an example listed for sale as lot 332 in Downies auction 316 in February 2014 and another example was listed at lot 72 in Noble auction 105 in March 2014.