You are losing something in translation.
"In NZ, the original purchaser when buying
the boat new will pay GST (goods and services tax - equivalent to VAT)"
Okay, what's that got to do with subsequent purchasers of the vessel? If the vessel was NZ flagged, surely they've got the records of that and you don't need to show them.
" but, unless that purchaser is a company which can claim the GST back as a business expense, any purchaser can't pass on the GST element of the total price
to subsequent owners."
I'm unclear what "pass on the GMT element" means. You mean, tey pass on the credit for havin paid the GMT? Since you're not a business, that wouldn't matter anyway, would it?
"But I was thinking that if, it had to be shown that GST had been paid then the only way to prove this would be to have a copy of the original invoice from the original owner."
Again, why would you think it had ot be shown that the GMT ever was paid? And if it ever was paid, again, wouldn't NZ know that simply because they had already previously flagged and titled it, which they wouldn't have done unless it was paid?
Are you just speculating, or has anyone *official* told you that you will need to show proof that GMT was previously paid on the vessel? And if they have told you that, surely they specified what they accept as proof?
Tax authorities generally are quick to actually list their requirements. Speculation is pointless, since they are not required to use the same logic that anyone else would use.