So she is your girlfriend.
De facto is a legal term in regards to a relationship in Australia and NZ. Thailand doesn't legally recognise de facto relationships, so under the law of the country that she is a citizen and resides, she is your girlfriend.
It may seem I am being flippant, but it makes a very big difference in a legal sense and that's what your are being judged on. Now just because Thailand doesn't recognise de facto relationships, that doesn't mean NZ won't recognise your in one, but you will have to prove it to the NZ requirements which are
How long you have been together; (3 years is the requirement in NZ)
Whether you share a residence (i.e. do you share a bedroom together);
Whether a sexual relationship exists between you;
Arrangements surrounding your financial affairs (e.g. are you financially dependent on each other; do you share bank accounts etc.);
Ownership, use, and acquisition of any property (assets or liabilities);
Whether you have a mutual commitment to a shared life together;
The care and support of any children; (even if not biologically yours)
Who performs various household duties;
How the public views your relationship (e.g. do your friends and family see you as a couple?)
She will have to apply for a normal Visitor Visa from Thailand, unless she has a very good job and money she won't get it on her own accord . So she will have to show that she is traveling with you, her relationship to you (if claiming its a de facto see above). that she has the funds to support her stay and that she a reason to leave NZ at the end of the trip.
To increase the chances of getting the visa you will need to be able to evidence the above in detail, not just claim its so.
Supply the right evidence and the visa will be easy to get.