That's pretty much my plan.get one extra sheet of ply - just in case.
I'm just hoping I don't have to order from your Brisbane supplier!
I've located one supplier of marine ply, about 30 miles North of me. No idea what he actually has on hand, though. I'll be looking about some more, I'm sure to find others.
What I do eventually buy will likely be based more on availability, rather than price. I'd not go so far as to say "Price doesn't matter," I will pay more for better quality.
With 6 oz. cloth for the hull and 4 oz for the deck (inside & out,) I'd think there's some room for discussion on exactly what goes in between. Again, as I said, it may come down to what I can find. I don't mind a drive to get it, but I'd not want to have to have it shipped. (Especially from your supplier, as good as he must be!)