Fast-food chain KFC has been fined more than $100,000 two years after a 16-year-old worker tripped and fell into a vat of hot cooking oil, suffering third-degree burns.

The teenager suffered severe burns to 9 per cent of his body, including his lower back, buttocks, right elbow and left shoulder.

The 16-year-old cook was working at the Eastwood store in Adelaide's inner-south when he fell into the tank of hot oil.

He was on only his fifth three-hour shift at the KFC outlet in the Adelaide suburb of Eastwood when he fell into the 51-litre tank, which was about half full.

He was one of three cooks and two trainees working in the store on May 15, 2015, and was responsible for flouring, cooking and "racking" chicken pieces.