A West Australian mother who was served chips from Chicken Treat that were seasoned with caustic soda will have her million-dollar payout halved.

Karis Louise Pringle, then aged 26, suffered caustic burns to her upper gastrointestinal tract after she brought chips to share with her nine-year-old son at a Bunbury store in May 2013.

She returned them to the store after noticing their strange taste before her lips began to blister and she started vomiting.

Ms Pringle was one of 11 people who suffered injuries from the contaminated chips that day and she was later diagnosed with post-traumatic stress disorder and a pain disorder.

In 2023 the WA District Court awarded her more than $1 million, which included compensation for loss of future earnings, medical expenses and general damages.

The judge found Ms Pringle had "abandoned hope of resuming a useful working life" and medical experts said she had suffered pain, discomfort and poor mental health.

But following an appeal from Tabloid, which operated the fast food outlet, the payout has been reduced to less than $517,000.

The WA Court of Appeal found the "primary judge failed to use, or palpably misused, her advantage as a trial judge" by awarding money based on the fact that Ms Pringle would ultimately earn about 40 per cent less for the remaining 31 years of her working life.

The court said the mother was able to continue full-time work for about 20 months after the caustic soda poisoning and had given birth to two daughters in the three years leading up to the trial.

On Thursday, the court revised the $350,000 awarded for loss of future earnings down to $90,000 and ruled that Ms Pringle's loss of future earnings would more realistically amount to about 50 per cent over a three year period.

The court said the amount awarded for future loss of earnings was a "wholly erroneous estimate of the damage suffered" because it was not likely Ms Pringle's future employment would be impacted until retirement age.

The court also revised past loss of earnings, superannuation and interest.