Support for same-sex marriage has caused a damaging rift in the NSW legal profession, after the Law Society came under fire for issuing a joint statement with the medical profession backing a change to federal marriage laws.

The NSW Law Society, which represents the state's 30,000 solicitors, joined with the Bar Association and the state division of the Australian Medical Association on August 19 to issue a statement expressing support for changing the Marriage Act.

Fairfax Media understands the Law Society's governing body, the law council, was divided over support for the change.

Fifteen of the council's 22 members were present for the vote on August 17 about whether to participate in the joint statement. Five members voted against it.