That is not correct. The registered owner of the vehicle is initially responsible for fine. In this case its a rental car company. If the registered owner was not the driver at the time, they must complete a Statutory Declaration stating who was responsible for the vehicle at the time the offense was committed, which they will state as being the person who signed the rental agreement.
So the vehicle renter is now responsible for the fine. The only way they can "transfer" the responsibility to another party is for them to complete a Statutory Declaration identifying the person to whom they had transferred the operational responsibility of the vehicle at the time the offense was committed, and that requires the driver to be identified by name, address and drivers license number.
In this case it is the responsibility of the person who has received the infringement notice (the person that the registered owner identified as being responsible for the operation of the vehicle at the time the offense was committed) to accept responsibility or to identify the driver. Stating you don't know who was driving is not an acceptable option in this case. If you don't know, then its your fine.
ok, so a slight difference between QLD and say VIC law.
under VIC law the registered owner would be responsible, then they would have to nominate the driver. they would of course look to the the rental agreement.
however the nominated driver then has to accept the nomination. if they don't know who was driving then they might not accept it.
the dispute may then come down to whatever the contractual agreement was between the rental company and the driver... which probably says something like 'you are responsible to pay all fines".
however the liability (who actually committed the offence) may remain undetermined and not attach to anyone.