Hardcastle won the silver medal in the 800-metre freestyle and finished sixth in the 400-metre freestyle at the 1985 European Championships. At the 1986 Commonwealth Games, she won gold medals in both the 400- and 800-metre freestyle. Her time of 8:24.77 in the 800-metre freestyle was the second-fastest ever recorded, 0.15 seconds outside the world record held by Tracey Wickham, and a new European record. Her time in the 400-metre freestyle was a Commonwealth Games record. She also won bronze in the 400-metre individual medley, and was a member of the team that won silver in the 4×200-metre freestyle relay. She won the bronze medal in the 400-metre freestyle at the 1986 World Championships.