Hi Cath, this is a male Myrmecia out looking for a mate. You can tell it is a male by the small head and rudimentary mandibles - he will die in a day or two, so doesn't need much in that department. In comparison, the queens have fully developed heads and mandibles as they live for years. By the black body and red legs it is likely to be Myrmecia fulvipes but I can't be sure. You're right about the petiolar node that sets it apart from the wasps, but as ants are derived from ancient wasps and Myrmecia is a fairly "primitive" ant, it can still give a pretty good impression of a wasp, especially the sting! Cheers, Jon