Kiribati, Nauru and Tuvalu are all independent countries - what they use for money is their business, not ours. They've all unilaterally chosen to use Australian dollars, but Australia doesn't produce any special currency for them, and they don't countermark our currency for use there. Just like American money used in Ecuador or Panama isn't specially marked.
Kiribati and Tuvalu have both made "circulating" coins in the past, in 1979 and 1976 respectively. To what extent they actually circulated, you'd need to ask a local, but as far as I know, they circulated side-by-side with Australian coins. These can be obtained fairly readily; particularly common are the recent issues of 1¢ and 2¢ coins - when Australia stopped making those denominations in 1991, both Kiribati and Tuvalu felt it still needed them, so ordered a couple of batches of their own coins made to fill the shortfall.
Nauru, however, has never made anything other than NCLT.
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