The Victorian Aboriginal Legal Service is bracing for a $500,000-plus funding cut as part of wide-ranging, national cost cutting to indigenous legal assistance services in the portfolio of Attorney-General George Brandis, QC.

The Preston-based legal service offers help to clients in Melbourne and six regional centres, but may have to reduce the number of lawyers and other frontline staff once the cuts go ahead, says acting chief executive officer Meena Singh.

"It's like we've learnt nothing from the Royal Commission [into Aboriginal Deaths in Custody]," said Ms Singh. "We're talking about the most vulnerable people, sitting in custody because we just won't have the time or money to see them. It's like trying to put a tiny bandaid on a really massive cut."

The legal service has an annual budget of about $4.3 million, and Ms Singh said it is an "extremely lean" operation, which relied heavily on the goodwill of its staff.

In Victoria, the indigenous custody rate has more than doubled in the past decade.

Aboriginal legal services across the country are already having to make decisions to withdraw or reduce key services ahead of proposed cuts and funding uncertainty.

On Wednesday, Children's Commissioner Bernie Geary and Aboriginal Children's Commissioner Andrew Jackomos joined with 24 other prominent people and organisations in an open letter calling on Prime Minister Tony Abbott, Indigenous Affairs Minister Nigel Scullion and Attorney-General George Brandis to urgently reverse any cuts.

They included the Australian Bar Association, the Public Health Association of Australia, the Australian Council for Social Services and the Law Council of Australia.

The letter called on the government to "take heed" of the Productivity Commission's recommendation that $200 million more be added to the legal assistance sector, including family violence prevention legal services. The Commission also recommended that cuts to indigenous legal services be reversed so that they could help more people.

Mr Geary told Fairfax Media that legal representation was vital for children, as any court appearance was a "life-changing" experience for them.

Free legal help was even more vital for indigenous children, who were particularly vulnerable to different levels of disadvantage, Mr Geary said. "For Aboriginal children the voice of the child is so much harder for the system to hear and we need to listen in a lot harder."

Meena Singh said many Aboriginal people were distrustful of mainstream services because of racism and historical experience, and the 30-year-old Victorian Aboriginal Legal Service offered a "safe space" to seek culturally-sensitive help.

About 40 per cent of its staff are Aboriginal and it has a unique program where locally-based Aboriginal community officers are available to visit people on remand and do welfare checks.

About 75 per cent of its clients have experienced family violence.

VALS lawyer Rose Cameron said her colleagues had large caseloads and sometimes saw as many as 25 clients in one day while they were out in regional areas.

Funding cuts would reduce the time she had to spend with complex clients who had mental health, drug and alcohol and other issues.

"It means greater levels of injustice when you're already dealing with people who experience injustice in every facet of their lives," Ms Cameron said.

Successive federal governments' contributions to legal aid commissions have fallen every year since 1997 from about 50 per cent to a third of all funding.

The Coalition scrapped Labor's promise to add $15 million to legal aid commissions last May. It also plans to cut about $43 million from the legal assistance sector, including Family Violence Prevention Legal Services, over the next four years.

The Age has contacted the office of Attorney-General George Brandis and is waiting for comment.