According to the ICAO document it's a Class C risk as assessed using Airservices Australia's own risk management framework. You are unlikely to find that published anywhere but according to the ICAO document it has four levels A, B, C and D. It states that A is unacceptable and D is acceptable but we need to guess about B and C. We can assume that C is tolerable as they weren't required to use the mitigation they had previously identified and we know the Chief ATCO was required to agree so C appears to be a tolerable risk if signed off at a senior level.
It is interesting that one ANSP identified a Class C risk and the other states that they never imagined that possibility in their hazard analysis. I can see that different safety management systems might give different interpretations of the risk but to not even consider the possibility suggests shortcomings in the hazard analysis process. I hope that is a one off and their hazard analysis is a bit more thorough for other operational risks.