That's because there is no reference because the ILS Z is not the preferred procedure. If it was, it would be written somewhere.
Well, technically, it should be. That is part of the Jeppesen naming convention for approaches. The preferred approach should be named the Z approach.
On any approach where there is currently a Y and Z option, and ATC hasn't nominated the Y or the Z, would in make any difference to what the aeroplane would do if you flew either?
In Australia, no, in other parts of the world, absolutely.
If I were flying into another part of the world where they had two separate approaches for the same runway, and I wasn't told which one to fly, I'd be asking! A close look at the Australian case and yes you can see there is no difference between the charts, so it makes no difference - but that shouldn't be a default position to take, as we are perhaps being a little unique in that regard.
The reason nothing is written about which to fly when told "cleared ILS" is because the controller should be clearing you for the specific approach - the ILS Y, or the ILS Z. That is what normally occurs when there are separate approaches for the same runway (for example, Japan). It is really no different to being cleared for an ILS vs NDB at MEL - they are different approaches. As are the ILS Y and ILS Z. At the moment, with only the DME the difference between the two, it doesn't matter, and we're assuming we're all on the same page. But it is an assumption that doesn't hold with the global reasons for having more than one ILS approach to a runway.