The Japanese investigated the possibility of land invasion of Australia in WW2, they believed that even if they made a successful landing it was going to be a logistical nightmare keeping their troops supplied which is why they opted for trying to control the water to blockade us. That is the only way Australia would ever be successfully subjugated by a non-nuclear option.
Most countries would suffer from sanctions of trade. Japan and China need us to supply natural resources and livestock etc. The draw back is if we did deny those countries our resources it would harm our economy as well.
In a way we are very lucky to have allies in the Commonwealth and the US to be a deterrent for countries like Nth Korea to push the button. Our major cities are miles apart too to make an invasion highly unlikely.
As I said in a previous post, the type of conflict that we could face would be protecting our fishing waters and ports and in the worst case urban warfare and terrorism in coastal areas.
When I was in the military there was a definite focus in special forces operations if things like this escalated.