Thanks everyone for your suggestions.
I'm actually a librarian at the State Library Victoria and this is a query that we received from a member of the public. It's really had us all stumped and although we've searched through copies of our electoral rolls and various relevant publications but haven't had any luck.
However, we recently got more information from the researcher and it turns out the record is from a Local Government Voter's Roll (From 1974).
It has an address listed and after the postcode appears P, LS and then another address.
At least this now confirms that the roll would have been created by the Victorian Electoral Commission, not the Australian Electoral Commission.
We only have a few copies of old Voter's Rolls in the library, but on a couple of them there are codes relating to the occupiers. I thought i might share the attached scan with you as it's really interesting.
We're now going to contact the VEC to see if they can provide any additional information.