Only a fairly small proportion of recent Australian veterans have seen real combat conditions. Even though tens of thousands have served in the recent wars, the vast majority have been in support roles - for the most part only our elite units have been routinely deployed to combat (not just special forces (SAS, Commando, etc), but even the 'regular' infantry units deployed have been the best of those - the Royal Australian Regiment, in particular). That could explain some of this - my impression is that the US has deployed a much wider range of forces into combat situations.
(Of course, just about any soldier or other serviceman can find themselves in combat but there is a different risk profile for somebody working in logistics and support, from somebody in a frontline infantry unit - it's all important work, worthy of respect, but it's not the same).
True but you don’t need to see combat to develop PTS. I think I’m going to do a little more research about Brits and Aussies. The American lifestyle is unique and may be causing more boots to get it.