Thanks for all your replies guys, it helps to understand the rules!!
So in fact, buying a second hand yacht in OZ needs to add the NZ GST to it, even if the yacht is not brand new...
I'm after a second hand cruiser/racer near 50ft. For the moment I focused on First 47.7 and in NZ they are none for sale (one from 36 degrees brokers but sold and was at 300k), while in OZ they are more and "cheaper". But of course, after adding the NZ GST, it's the same price...but at least there are options