WESNET national chair and Annie North chief executive Julie Oberin said parliamentary support for the Family Court's merger with the Federal Circuit Court was "beyond belief".

Ms Oberin was among more than 155 signatories to an open letter opposing the reform.

The signatories, including retired Family Court and Federal Circuit Court judges, warned the merger could undermine the safety of family violence victim-survivors.

Their concerns were echoed by central Victorian agencies involved in the family law system, including the Centre for Non-Violence, Loddon Campaspe Community Legal Centre, and O'Farrell Robertson McMahon Lawyers. The Bendigo Law Association was also opposed to the merger.

A proposed law passed the Senate overnight and received federal parliament's final approval on Thursday.

Ms Oberin said the politicians who supported the bill had not done so in the best interest of those who used the courts.

"With this merger there will be more harm done to children, victims of family violence and families," she said.

"The court will now be tasked with doing so much more with fewer resources."

She anticipated lengthy delays and less expertise from the court.

"It is beyond belief that they completely ignored the consistent advice of highly respected experts," Ms Oberin said.

She said the so-called benefits of the merger were based on a widely discredited report.

Centre for Non-Violence chief executive Margaret Augerinos acknowledged the intent of the reform was to strengthen systems.

"We do not believe that will be the outcome of this decision," Ms Augerinos said.

"We are disappointed the government went ahead against the advice of experts in this area."

Increased specialisation and investment in court systems would have been the organisation's preference for reforms to the family law system.

"No court system is a perfect system, but the Family Court is a specialist court where judges have the training and systems have been developed to maximise safety for women and children and to deliver good and just outcomes for people using family court services," Ms Augerinos said.

"A specialist focus is required in these often high-risk settings, where safety must come first."

Accredited family law specialist and O'Farrell Robertson McMahon Lawyers director Marika McMahon said questions remained about the appeal process.

She believed the existing court system would have been the ideal one if it were adequately resourced.

Ms McMahon intended to continue advocating for the courts to be adequately resourced for family law matters, including more judges, more online resources, and more supports like counsellors and psychologists available in court.

She said the delays and complexities the Family Court experienced were often as a result of a lack of resourcing.

"I think it's really sad," Ms McMahon said of Thursday's developments.

"We've got a history of a family law court that was groundbreaking. Now, with a stroke of a Senate pen, that's been done away with by people thinking that's going to change the system."

In a joint statement, the Law Council of Australia, Women's Legal Services Australia, Community Legal Centres Australia and the National Aboriginal and Torres Strait Islander Legal Services said the merger would land judges with an impossible task.

"The government and new court will be under heavy scrutiny to deliver court efficiency, resolve 8000 additional cases, reduce costs, reduce the time separating families will spend before the court and reduce delays, even allowing for the impact of COVID-19," the statement said.

- with AAP