The Barr government's proposed compulsory third party insurance scheme has already been attacked by unions for handing insurance companies too much power in the wake of the banking royal commission.

Lawyers also say the scheme will severely reduce compensation to road traffic victims.

But even Insurance Australia Group - which under the NRMA brand underwrites 164,000 CTP insurance policies in the ACT - has described parts of the process that will determine who is eligible for extra compensation as unfair.

While anyone injured in a motor vehicle accident will be able to access treatment and care for up to five years regardless of fault, only those that pass a 10 per cent whole of person impairment threshold can sue for extra benefits.

The company told an ACT Legislative Assembly inquiry into the proposed scheme it was concerned about the complexity of the assessment process.

"Injured people will be required to take in complex information upon which they will be required to make decisions that have potentially significant financial impacts upon them. All of this while they are dealing with the effects of their injuries," its submission said.

"It is also likely that the complexity of the process will cause injured people to seek the assistance of a lawyer, for which no legal costs may be payable by the insurer."

The company said it was particularly concerned by the requirement for injured people to pay an excess to the insurer for the assessment where the insurer considers there is no permanent impairment.

"We consider that this requirement disadvantages those injured people of lesser financial means. We also submit that enforcing this process and collecting/recovering excess amounts would be administratively difficult and expensive, to say nothing of the impact on the relationship between the injured person and insurer," its submission said.

The excess is likely to be set at $500 or one-quarter of the assessment fee, whichever is higher.

Asked whether the changes were fair, a government spokeswoman said the cost of a whole person impairment assessment could be in the thousands of dollars.

"An excess payment for the independent assessment is only required where the injured person requests the independent assessment even though they are not considered to have any likely permanent impairment as a result of their accident," she said.

"The excess payment is fully refundable if the injured person’s whole person impairment assessment by an authorised independent medical examiner determines the injured person has some permanent impairment (eg more than 0 per cent)."

The insurance group also hit out at the decision to cut income benefits for injured people who were at retirement age plus 26 weeks.

The clause would mean an injured person who was older but working would receive no income replacement if they were unable to work because of it.

"This is an unjust outcome for injured people who continue to work past retirement age by choice or need," the company said.

The government spokeswoman said the cut-off was consistent with statutory motor accident schemes and workers compensation schemes in other jurisdictions, and had been included in the model chosen by the citizens’ jury.

"At this point, people can access the Commonwealth age pension and/or their superannuation benefits to provide income," she said.

"Older people with more serious injuries, and are not at fault, can still make a common law claim for damages including for any lost income."

The scheme would also prevent injured people from claiming damages for lost earnings in the first year after the accident.

The provision seems geared to prevent duplication, as there are income replacement payments available.

But Insurance Group Australia told the inquiry it would mean there were people who were out-of-work for reasons like unpaid maternity leave would be locked out of recovering those lost earnings forever.

It would also leave injured people out of pocket on superannuation.

"We consider this outcome to be inconsistent with the citizens' jury objectives," the insurer said.

The government spokeswoman said the requirement that injured people receive defined benefits for loss of income for the first year after an accident had been a design feature of the jury's chosen model.

Hearings about the new insurance scheme are scheduled for Monday and Tuesday, with lawyers, insurance companies and unions to give evidence.