I don't collect Papua New Guinean stuff myself so haven't come acquired any of its Franklin Mint struck coins, but it wouldn't surprise me.
Further on the Cook Islands coins though, the 1977 10t and 50t pieces seem quite plentiful on ebay. Also, I'm told that at least towards the end, the Franklin Mint stuff was made to order i.e. you'd register your interest then when the interest period had closed sets were produced. No idea what sort of contract they had with the governments but it could well have been that the governments ended up with the surplus/cancelled/unpaid for coins, and with nothing better to do with them they just circulated them.