HI Cath, not the correct ID, but you are quite right that they look a lot like Leptomyrmex, with the reddish head and long-looking legs. Iridomyrmex purpureus don't normally look quite like this, but you have seen individuals from two different nests in the middle of a dispute over territory or a food source, and they make these very characteristic body poses, trying to look as big as possible. There is good information on this included in the Wikipedia page for these ants, quite a long way down the page under the heading "Ritualised Fighting", where you will see a photo just like yours! Cheers, Jon