As a child Alva Colquhoun suffered from asthma, with a doctor recommending that she should take up swimming to help with controlled breathing. Colquhoun was only five-years-old at the time, with her progressing to become a competitive swimmer in her teens, before being selected to represent the Australian national team. In 1956 she was named as an alternate for the 4 × 100 metres freestyle relay at the Melbourne Olympics, but did not ultimately compete.
Two years later Colquhoun was named in the Australian swimming team for the British Empire and Commonwealth Games in Cardiff, Wales. She won an individual bronze in the 110 yards freestyle, along with a team gold with Dawn Fraser, Lorraine Crapp, and Sandra Morgan in the 4 x 110 yard freestyle relay. In 1960 Colquhoun returned to the Olympics for the Roma Games, swimming the anchor leg in the final of the 4 × 100 metres freestyle relay to help Australia win silver. At the Games she also helped to solve an infamous dispute in a team meeting of who was going to swim which leg in the medley relay. After her retirement Colquhoun lived in rural Victoria, where she taught swimming to 100s of children.