They are missing as when GMH had the HQ Service/Warranty report copied to microfiche on 12th November 1974 someone has missed a box. So anything after L265730 on the end of page 4198 of the printout was not microfiched. This doesn't just include the end of Elizabeth HQ but also all Mosman Park HQ.

There would have been two records. What was in the IBM360 itself, which contained all of the information for every vehicle. That would have all been stored on magentic media. This was the Vehicle Master Listing.
At the wrap up of a Series they printed a small subset of that data for service and warranty. In database terms its called a report, and it was indexed by PSN. If you look at the HHS website there is a copy of page 1 of the Dandenong HQ section, you can see its tractor feed IBM1403 or later printed on z-fold paper. The first half are error or incorrectly entered details from HK-HG. This paper record would have been kept at head office. You can see by how tatty the end sheets can be that these records hang around for a while. At some stage it was microfiched, and I have never heard of the paper records that were microfiched existing past that point. Given there are a half dozen known copies of those microfiche I reckon each State Zone office had a copy. As they all come from the same source none have the last box (or boxes) of records.
By mid 1978 GMH switched to an outsourced industry wide system called “Oneline Warranty Reporting” or something like that. They transferred everything to them and that is when the paper records ended. HZ ends about June 1978 and that is about when the HZ report was printed. Not sure how they ended UC.