Unfortunately, "going to a dealer" in tropical north Queensland can be a little tricky, but Cairns is better situated than most other regional cities; there are two dealers listed in the Yellow Pages there but I've never been to Cairns so I can't comment on them. Your next nearest dealer is in Mackay, 730 km away. I have been there, and while I'd vouch for him being honest enough, he doesn't tend to offer anywhere near 75% for non-Australian coins, simply because very few of his customers up there want foreign coins. I suspect any shop-front dealership in Cairns would be similar.
Your best bet at not being "ripped off", in terms of getting full value for the coins, would be to sell them yourself on
ebay. This, however, takes a lot of time and effort on your part, which you may not be able to spare.
Your other option, especially if there are a large number of coins and/or there are valuable coins amongst them, is to contact one of the big coin auction houses down south (either in Brisbane or interstate) and consign with them; some of these auction houses make tours of regional Australia to collect items so you might not have to pay much for shipping. The disadvantage with this method is having to wait for their next auction before you are paid.
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