Legal firm Slater and Gordon practice group leader Martin Carrick told an inquiry into the reforms that the new scheme would cut payments by more than $100,000 in some cases by stopping victims accessing compensation for damages through the courts.

Mr Carrick has previously told MLAs many people with ongoing injury and incapacity for their pre-injury work would be denied entitlements after five years, and many would be financially devastated by their reduced working capacity. Others would be thrown onto welfare payments, he said.

At a hearing on Tuesday, Mr Carrick said the planned scheme - subject to a Legislative Assembly inquiry - had the potential to create unfair decisions that harmed people who had been injured.

"This system goes such a long way down the track of putting injured people in the hands of insurers," Mr Carrick said.

Changes proposed to the scheme emerged from an ACT government experiment asking ordinary people or a "citizen's jury" to solve policy questions.

Mr Carrick said the "jurors" had been led to select the proposed scheme, from a choice of several options, by someone with a predetermined view about the right outcome. While he didn't oppose coverage for people deemed at-fault in road incidents, he believed this did not require a new scheme.

"That can be done without doing such dramatic harm to the system as it is now, and reducing benefits to people who are in the system."

He has previously told MLAs the scheme's changes would allow insurers to determine medical treatment available to injured people and the lost income to which injured people were entitled.

Mr Carrick on Tuesday told the committee investigating the proposed scheme that insurers would certainly decide injured people had earning capacity, even if their condition prevented them from finding work and receiving another income.

"I can tell you from personal experience with people working in this area that it's very difficult for people with injuries to find jobs," he said.

The decisions could hurt people's finances and stop them making mortgage payments, Mr Carrick warned.

The ACT government has defended its planned reforms to the scheme, promising a commission for motor accident injuries. Slater and Gordon says insurers can't be trusted despite rules, pointing to revelations about their behaviour in the banking royal commission.

"We do not accept that guidelines or protocols will effectively prevent insurers from acting in their own interest," Mr Carrick wrote in his submission to the inquiry.

Treating doctors, and not insurers, should decide what is a reasonable and necessary medical treatment for injured people. Mr Carrick also warned the new scheme could invite insurers to pay medical providers to supply them cheaper services under cost-saving arrangements, to the detriment of injured people receiving treatment.

He said insurers appeared delighted with the ACT government's planned changes to compulsory third party insurance, something that should make ordinary people suspicious.

The inquiry, expected to report by December 14, has received more than 70 submissions commenting on the changes to the ACT's scheme.