Which is one of the reasons ADS-B is not a regulatory requirement for ADF aircraft.
ADF aircraft without ADS-B are permitted to fly in airspace in which civilian aircraft are not allowed to fly if they don't have ADS-B. Which gets us back to Dick's point.
Either it's sufficiently safe for an aircraft without ADS-B to be in a volume of airspace, or it isn't. The objective risk created by the aircraft while in a volume of airspace is the same whether the aircraft is owned by the Commonwealth of Australia, the Commonwealth Bank of Australia, or Joe Citizen. If the risk is sufficient to require the aircraft owned by Joe Citizen to be fitted, despite the cost, the risk is sufficient to require the aircraft owned by the Commonwealth to be fitted, despite the cost.
All of which goes to show that the mandate has little to do with objective risk or objective cost/benefit.