Hardcastle initially retired from competitive swimming in 1986 at the age of 17; in a 2012 interview she said that after years of training she had wanted to have a normal teenage life. After a spell working as a secretary at Ford, she decided to return to the sport in 1992 and resumed competing in 1993. Hardcastle was a member of the relay teams that won bronze in the 4×200-metre freestyle at the 1993 European Championships and silver at the 1994 Commonwealth Games, where she also won an individual bronze medal in the 400-metre freestyle. In 1995 she won the 800m freestyle gold medal and bronze in the 400m freestyle at the World Short Course Championships. At the 1996 Olympics in Atlanta she reached the final of the 800-metre freestyle, finishing eighth.