Hi there,
Good question, I was stuck on the same thing.
My textbook states that this is an element of uncertainty with the referral of powers, as to whether it then becomes exclusive. Apparently these questions have not been tested in court as the need has never arisen whereby the states need to take back or change the referral of power.
I hope this helps.
Haha, do you have my textbook??!! (CPAP)
Yes, that's basically it. Because of the wording of s51, and the fact that a referral comes under s51, we assume it becomes concurrent. And HCA obiter supports that. BUT it's theoretically possible that a state could pass a referral *saying* it's exclusive - what then? Who knows. Also, you can argue that, because the s109 inconsistency rule applies to referrals, they basically become exclusive in an everyday practical sense, if not a legal one.
Legally, though - probably concurrent.