freitasm: If they have a registered New Zealand company then they will have to collect GST. What is the situation? Search the Companies website for the company details.
Gooseybhai: Was this for invisible services or actual goods? i.e. where was the goods shipped from if physical goods?
You say the invoice says 'includes applicable taxes'. Wouldnt this be their way of saying GST inclusive?
Your accountant (or yourself) can eaisly workout the GST on this invoice i.e. workout 15% of the total.
Is their tax number (or anything else that resembles a tax number from a different country) on the invoice?
Gooseybhai: Agree. Have you checked with the ird regarding doing business with off shore companies?
Most likely a common question they may cover in their faq.
It might be beneficial if you asked them for advice before they ping you for something you didn't know.