The Law Society says the same-sex marriage bill as it stands could criminalise church ministers who refuse to wed gay couples.
The Marriage Amendment Bill is being considered by a select committee.
Labour MP Louisa Wall, who is sponsoring the bill, says it will not change the Marriage Act, which makes clear that celebrants have no obligation to wed any couple - gay or straight.
Section 29 of the act says a marriage licence authorises but does not oblige a celebrant to solemnise a marriage.
But the Law Society says celebrants may still be bound under human rights guidelines introduced after the Marriage Act.
Paul Rishworth, the society's spokesperson on law reform, says a provision should be included in Ms Wall's bill making it clear that celebrants would be able to turn away same-sex couples if they chose.
However, the Human Rights Commission maintains the bill will not criminalise ministers who refuse to wed gay couples.
Chief Commissioner David Rutherford says the provision is not necessary.
"Religious freedom of ministers protects them and enables them to choose who they marry, whether that person is straight or gay, Muslim, Christian, Jewish, Hindu, Sikh - whatever. Religious people have the right to choose who they marry in their church."
Mr Rutherford says any legal challenge would be decided by balancing different human rights against each other.
Lobby group Family First has previously argued that the bill could criminalise ministers. Its director, Bob McCoskrie, wants the bill thrown out.
"We think, ultimately, the bill should just be voted down. It's far greater that just this one issue, but if they don't understand the legal implications in this one area, then there's lots of other areas around the social and legal ramifications that they may not be aware of as well."