Motorists were originally estimated to save between $91 and $171 as part of the proposed scheme, which would entitle everyone injured in a motor vehicle accident with up to five years of medical treatment, care and lost income, as long as they were not breaking the law at the time of the accident.

This is vastly different than the current common law system where injured parties must prove their crash was someone else's fault in order to get a payout (although people can apply for up to $5000 for medical expenses without having to prove someone  else was at fault).

To constrain the costs of the proposed scheme, only people with an injury severity rating of more than 10 per cent whole person impairment were going to be allowed to pursue damages through the legal system. Modelling based on data at July 1 2017 put the estimated passenger vehicle premium of the new model at between $385 and $465.

But the draft legislation has been changed to grant an exemption to injured children who still need medical treatment four years and six months after their accident to seek compensation through the courts, even if they do not meet the 10 per cent threshold.

An exemption has also been added for injured workers, who don't meet the threshold but have been unable to retrain or find new employment, so long as an independent medical assessor deemed them incapable of work.

Injured adults who didn't meet the threshold but still needed medical treatment after five years would also be able pursue a lump sum payment from their insurer through ACAT under the changes. This would only apply if they were not-at-fault in the crash.

As a result of the changes, as well as the most recent premium filings under the existing scheme, new costings show the estimated passenger vehicle premiums are between $425 and $510.

That's a saving of between $14 and $99, based on premiums as of March 18, 2019.

An ACT government spokeswoman said these figures were estimates only.

"The bill requires the licensed private sector insurers to set their premiums and have these approved by the Motor Accident Injuries Commissioner," she said.

"As such, the actual premiums charged could vary from this estimated range if there are further changes to the legislation or other factors that must be taken into account at the time premiums are determined."

Canberra motorists currently pay some of the highest CTP premiums in the country, a fact Chief Minister Andrew Barr cited when he announced a citizen's jury on the topic in 2017.

One of the constraints placed on the jury was that premiums could not rise as a result of the deliberative democracy process.

Greens crossbencher Caroline Le Couteur said a reduction in premiums was not their number one priority.

"From our point of view it has always been we want a fair system," Ms Le Couteur said.

"Under our current system if you're at fault you don't get any money and that means with the new system an extra estimated 600 Canberra people a year will get compensation from CTP and we think that is a major plus for Canberra drivers."

The amended bill will be introduced to the ACT Legislative Assembly on Tuesday after weeks of negotiations between Labor and the Greens.

The Greens had threatened to oppose the legislation if changes were not made to address certain "harsh outcomes" of the new scheme.

Unions had also strongly opposed the changes, and launched an advertising blitz hitting out at the power given to insurers under the new scheme.