The woman was 19 when she suffered whiplash and impact injuries to her right side when her car was hit head-on along Clive Steele Avenue, Monash, in February 2005.

Much of the damage healed without causing disabilities but the injury to her right ankle continued to cause significant pain and mobility issues.

The painkilling medication she was prescribed for her physical injuries caused lasting damage to her bowel.

She also developed an adjustment disorder, with mixed anxiety and depressed mood.

She sued and was awarded $599,586 by an ACT Supreme Court judge in July 2012.

But the driver at fault went to the ACT Court of Appeal, arguing the primary judge had given the woman too much money in damages for loss of past and future earning capacity and future out-of-pocket expenses.

Three appeal judges - Justice John Burns, Justice John Dowsett and Justice John Nield - found no errors in the award for past earning capacity and future out-of-pocket expenses.

But Justice Burns and Justice Dowsett found the judge had erred in assessing future earning capacity. They ordered damages to be reduced to $575,411.

"The appellant has succeeded on one issue only, being failure of the primary judge to allow in her calculation of damages for deferral of a portion of the damages awarded for future impairment of earning capacity," Justice Burns wrote.

"This error was conceded by the respondent, albeit not until shortly prior to the hearing of the appeal.

"On each of the other grounds of appeal, the appellant has been unsuccessful."

Despite the win, the judges ordered the at-fault motorist to pay four-fifths of the woman's legal costs for the appeal.