I personally haven't imported anything like this (I'm NZ based) but if I were you, I would ask my supplier if they know of any of their clients/retailers who have had problems.
I would also give customs a call and see if there are any laws you need to know about or licenses you need to import them (boring advice, I know, but it's always best to be safe than sorry!).
If you import these, they are likely to be inspected (as are most shipments that go into Australia). If they are viewed through an X ray machine, then you can bet your last dollar that they will do a pretty thorough inspection. However, it won't take long for them to figure out that they are merely paintball guns.
Unless there are any local laws that you have violated, I would say that worst case scenario, your shipment might be delayed for a couple of days to be inspected.