I imagine there must be quite the black market?
'Australia has some of the strongest firearms controls in the world.
However, criminals use the grey market and dark web to traffick illegal weapons for use in criminal activity. This is a serious national threat and a significant safety concern for the Australian community.
Weapons trafficking has links to:
aviation crime
money laundering and financial crime
serious and organised crime
terrorism.
The Australian Criminal Intelligence Commission (ACIC) estimates more than 260,000 firearms are in the illicit market.
The market for illegal weapons is driven in part by:
drug traffickers
organised crime rings
outlaw motorcycle gangs
terrorist groups.
While these groups have direct links to illicit firearms, other criminals get and use firearms illegally through various means.
Some weapons were not handed in as part of the buyback scheme when Australia tightened its gun laws in 1996. Some are stolen from firearm dealers. Some are illegally manufactured, even using 3D printing. They may also be illegally imported.'
Source:
Illegal firearms and weapons | Australian Federal Police.
Basically unless you are a member of organised crime they are generally prohibitively expensive for the common folk in my view