hope you don't mind if I jump in;
while once upon a time the size of a ply was standardized and an 8 ply might have actually been made of 8 singles, nowadays anything that fits the accepted weight of an 8 ply yarn is called 8 ply, regardless of how many singles make it up.
I think though, that there may have been a little drift over time, as I have bought vintage/older stuff called 4 ply (or whatever) that isn't actually the same as what I buy commercially new.
I think everything should be sold in WPI- at least it is standard.
Sorry I didn't reply before, But 'ply' isn't just the amount of singles in Australia, its the weight, just like worsted or whatever. It doesn't really have any bearing on the number of singles. For instance, I am knitting with a 8 ply weight yarn, but it is a single. On the net, I have heard of the ply system being termed "yarn weights 1, 2, 3 etc"
I guess its just differing meanings.
And I totally agree about the WPI measurement, it would take all the guesswork out!