Useful recent Australian tax case on GST and vendor loans.
Mathiesen Family Trust v FC of T  ATC 10-327, 15 July 2013.
The taxpayer agreed to sell land for $3.5m. The purchaser could only pay $2m on settlement and borrowed the balance of $1.5m from the vendor, secured by a mortgage over the land.
The vendor argued GST should be accounted for on only the amount received and in substance the financing arrangement was a deferral of payment of the purchase price.
The Court didn’t agree and decided the vendor did receive full consideration for the sale on settlement. It held the loan from the vendor was a separate transaction that permitted the purchaser to complete the purchase of the land. GST was therefore required to be paid on the full sale price including the amount of the loan.
I think a NZ court would reach the same conclusion if presented with the same facts.
In practice of course many land sales between NZ GST registered entities are zero rated so it’s not really an issue.
However, it’s something GST registered vendors selling land to unregistered entities [e.g. a residential developer selling to a private individual] need to be wary of whenever vendor financing is part of the deal.