There's information about marriage and transgender people on the Department of Internal Affairs (DIA) website and in the Human Rights Commission's publication Human Rights in NZ 2010.
Declarations and the wider implications
It is important to recognise that the declaration does not apply to all areas of life or the law (section 33). It only applies to the applicant’s birth certificates (section 28). If another area of the law has a different test for sex, showing the sex on the birth certificate may not be enough to meet that test. One example is marriage. Marriage is available to opposite sex couples but a declaration that the birth certificate should show a particular sex does not automatically mean that person can marry in that sex.
The High Court held in Attorney-General v Family Court at Otahuhu  NZFLR 57 that it is possible for a transgender person to marry in the nominated sex. However, the Court held that the person must have reached “the end of the continuum” in terms of the transition process. Michael held that the test to change a birth certificate can potentially be met before the end of the continuum is reached, so a person might be able to change their birth certificate but be unable to marry in that sex if the test in Otahuhu is not met.
It is not clear from Attorney-General v Family Court at Otahuhu exactly what surgery is required to reach the “end of the continuum”. The judge held that a male-to-female transgender person would have had “the penis and testes removed, and have had a vagina-like cavity constructed, and possibly breast implants". A female-to-male transgender person would have had “the uterus and ovaries and breasts removed...and possible (sic) a constructed penis" (at ).
Because a declaration under section 28 is not a declaration for all purposes of the law it is also possible to marry in the opposite sex to that recorded on the birth information without getting a declaration from the Court (if the Otahuhu test is met). To do this the person would need to provide sufficient medical evidence to the Registrar when applying for a marriage licence.
The last sentence means that someone who has had enough surgeries to marry as a man or as a women doesn't necessarily need to get a declaration to change their birth certificate too.
Section 30(2) of the Births, Deaths, Marriages and Rela-tionships Registration Act 1995 prevents a trans person who transitions after marrying from changing sex details on their birth certificate. This requirement reflects the current prohibition on same-sex marriages. As a result, a trans person is effectively required to take the preliminary step of dissolving their marriage or changing it to a civil union.
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There are also restrictions on whether a trans person who has changed their sex details on a birth certificate is able to marry as that sex. In Attorney-General v Family Court, the High Court held that it is possible for a trans woman to be recognised as female and marry a man, and for a trans man to marry a woman. However, the legal threshold in this 1995 High Court case was that the person must have reached ‘the end of the continuum’ in terms of their physical transition process. The judge held that the minimum requirement would be vaginoplasty for a trans woman and a full hysterectomy and mastectomy for a trans man. This threshold is likely to exclude the majority of trans people from the right to marriage.
Thanks to Jack for gathering this information and explaining it.