For the Australian market, only the 12.9" 128GB (wifi and cellular) are available for May delivery (24th - 28th). All other 12.9" models are expected in June (1 - 16) at this stage. Like iamMacPerson said, all 11" models are still available for May delivery (24 - 28) in Australia.
I am not sure if Apple is surprised by the popularity of 1TB and 2TB models (at least at this early stage), but I am personally astonished. I want 16GB RAM models too but, I personally cannot justify 50% to 80% price hike (of course, that increased the capacity too - which, I don't need at this stage). Maybe things will look different once the new iPadOS is launched and iPads can run MacOS apps or something along those lines. I would be interested to see Boot Camp-like (but improved) system where the user can decide to boot-up their iPad with iPadOS or MacOS. That way, folks can turn their iPads (with keyboard) into a touch-screen laptop like a Surface Pro.
Apple likely manufactures the 1-2TB configurations at lower quantities versus lower storage tiers so quicker sellout.