The Celebrant Association says it supports a crackdown on celebrants who sign off on marriages at which they don't officiate.
The Department of Internal Affairs has written to marriage and civil union celebrants warning the practice is illegal and saying it also puts the validity of the marriage or civil union at risk.
Association president Wanda Brittain says she has been approached several times by a non-registered celebrant to sign off on a ceremony she didn't perform.
She says the practice is growing because of restrictions on celebrant numbers.
The Registrar General of Births, Deaths and Marriages says he has heard of a number of cases where a non-registered celebrant conducted a ceremony and had a registered person sign off the paperwork.
He says both celebrants in such a case are risking legal action.