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Uca coarctata
Yep! It seems like the right ID. Well actually I saw these guys are the river part right next to the botanical gardens. So there is a mangrove forest.
Thanks for the ID!
Your location (and the fact that I'm also on a shore) really helped me with this. If you haven't - go ahead and try the "Bad Request" link. It should take you to the Queensland Museum's page on the crabs. :)