Cotton On is only selling the 4 rainbow ponies they have for sale at the moment.
In the past, sometimes "exclusive" ponies do pop up later through other retailers- the exclusivity is likely to be for a limited time. That was true for the ToysRus and Target "exclusives" in many cases.
So I wouldn't give up hope!
(Also just FYI the cheapest shipping for first class to AU or NZ appears to be $23.25) - a ridiculous price, I think worth it to wait and see what happens, since the ponies have only just started to get stocked with any regularity in the US>