Rex Patrick has agreed to back the merger after the government promised to ensure a minimum number of specialist family law judges.

Senator Patrick denied the changes amounted to dissolving a superior stand-alone court.

He said the legislation simply took the three jurisdictions dealing with family law and put them under the one roof.

"That will create efficiencies - it does nothing to dilute the expertise of the court," he told AAP.

"It's not a major departure from where we are right now."

Senator Patrick said there would still be a specialist "division one" with Family Court judges and a specialist appeals court.

Legal experts, community organisations and 13 retired judges published an open letter on Tuesday in a last stand against the merger.

Law Council of Australia president Jacoba Brasch said it was not the time to proceed with an "unnecessary, risky" plan opposed by all non-government members of the lower house.

The Law Council, Community Legal Centres Australia, Women's Legal Services Australia and National Aboriginal and Torres Strait Islander Legal Services were among more than 155 signatories to the open letter.

"Safety must come first in family law," Women's Legal Services Australia spokesperson Angela Lynch said.

"Our opposition to the proposed merger of the family courts is centred on ensuring the safety and best interests of the child and the safety of adult victim-survivors of family violence in family law proceedings."

Laws to crunch the Family Court and the Federal Circuit Court into one were first introduced in 2018, but lapsed when an election was called and have been languishing in parliament since being reintroduced in 2019.

Community Legal Centres Australia chief executive Nassim Arrage said the merger would move away from a specialist family court model, exposing family violence survivors to unnecessary risk.

Both Labor and the Greens are concerned the bill will spell the end of specialised judges for family law, particularly in domestic violence cases, and result in worse outcomes for women.

Now counting on independent senator Rex Patrick and One Nation to support the changes, the Morrison government is expecting the Federal Circuit and Family Court of Australia Bill to pass.

Attorney-General Christian Porter says there will be a single point of entry for the federal family law jurisdiction and, ultimately, a common set of rules, procedures, practices and approaches to case management.

Australian Associated Press