@Andrew *****
You are indeed correct, I just got off the phone with the consular services section of the New Zealand Embassy and they DO issue the affidavit of income from abroad notary letter.
If you're in Bangkok you make an appointment, show up, fill out the letter (you are NOT required to show proof that what you're putting on the letter is backed up with anything BUT you can if you want to) and they will notarize it
That notarized document from your consulate is all you need to show proof of funds for getting visa/extension inside the country. (you need to state a minimum of 40K baht a month for marriage/raising thai children and 65K baht a month for retirement).
Good luck with it, you are one of just a handful of countries that still issue that affidavit here in thailand (y)