She starred in several events, and was thus a natural competitor in the pentathlon (which was replaced by the heptathlon in 1981). In 1978, she qualified for the Commonwealth Games, but could not compete because of an injury.
By the time of the next Commonwealth Games (1982), she had married Chris Nunn (a decathlete). In the first heptathlon competition at the Games, she upset the English favourite and took the title. At the inaugural World Championships a year later, she was placed 7th.
Because of the 1984 Summer Olympics boycott, Nunn was one of the medal candidates for the Olympic title, too. The competition was incredibly close, with five athletes fighting for the medals. After the competition, there was confusion about who had won, but when the smoke cleared, Nunn had scored 6390 points, five more than runner-up Jackie Joyner-Kersee. In addition to her gold medal, Nunn was also placed fifth in the 100 m hurdles event, and seventh in the long jump.
After the Olympics, Nunn abandoned the heptathlon, and switched to hurdling. She was hampered by many injuries, but managed to win a bronze medal in the high hurdles event at the 1986 Commonwealth Games. She quit sports in 1990.