Hi Alison, I think this is Anonychomyrma rather than Iridomyrmex. They can be hard to tell apart, but the fact that they have a nest in a crack in a tree is a very good indicator of Anonychomyrma, which are largely arboreal. Iridomyrmex almost always make nests in the ground. A similar sighting is below. Cheers, Jon Anonychomyrma sp. (genus) (Black Cocktail Ant)