Conservative lobby group Family First NZ says a proposed change to birth certificates prioritises "ideology" over biology, calling the change "disturbing".
The gender listed on someone's birth certificate might soon be able to be changed with a simple statutory declaration from the person it belongs to, rather than a lengthy process involving the Family Court.
Guardians will also be able to complete a statutory declaration to change a child's listed gender.
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Bob McCoskrie, national director of Family First, said this should not be allowed.
"By choosing your own gender in your birth certificate, the certificates will become an object of unscientific gender ideology and effectively tell medical professionals that they got it wrong at time of birth. Circumstances may change but a historical document should not be able to be changed."
He also questioned the number of genders that would be allowed.
"Facebook have more than 70, and counting."
And said "the overwhelming majority" of children "grow out of it" by the time they hit puberty.
In contrast, the suggested changes have been welcomed by groups that represent trans and gender minority groups, as well as the Human Rights Commission (HRC).
"A person's gender identity is one of the most intimate areas of a person's private life," says Taine Polkinghorne, HRC advisor for sexual orientation, gender identity and sex characteristics.
Family First has a history of opposing changes to laws and regulations regarding gender and sexual orientation, such as the legalisation of gay marriage.