Closest thing I can find to an answer on the customs site is this, it goes on about importing normal ammo and permits blah blah then says.
Please Note
The above information does not apply to specialised ammunition (and components) such as the following:
• Armour piercing
• blah blah blah...
To import these types of ammunition, written permission to import must be obtained from the Commonwealth Attorney-General’s Department prior to importation.
So doesn't rule it out? Legal?
Just curious, not planning an armoured car heist or anything