Hi Matthew, nice photo. I had to do some work for this ID as two species of Myrmecia are quite similar - M. simillima and M. forficata. I've IDed the ant as Myrmecia simillima as the mandibles are straight and even width, particularly the last quarter of their length back towards the head. The generally similar Myrmecia forficata has slightly curved mandibles which have a wider section near the head. Luckily, your photo shows the mandibles very clearly! Cheers, Jon