Did you read the "small print" of your agreement with Alamo ? It is normal for car hire companies to charge admin fees when dealing with parking and traffic "citations". These can be from councils, Transport for London, or private parking companies. It is all legal, I'm afraid.
If they do, indeed pass on your Australian address to the council, the council will have great difficulty in pursuing the PCN penalties. Debt collection can only take place in the UK. However, I suspect Alamo will pay the PCNs and then off-charge you.