Dennis' sister Thora developed quickly into a competent competitive swimmer, and joined the Sydney Ladies' Swimming Club and was later selected for the 1928 Summer Olympics in Amsterdam, but was prevented from going on the grounds of being too young to travel alone overseas.

Dennis pleaded with her father to let her join, which he agreed to on the condition that she first swim the 33 yards (approximately 30 metres) across Clovelly Bay.

Due to financial difficulties caused by the Great Depression, Dennis managed to attend the Olympics only after assistance was rendered by the Police Department for travel and uniform costs.

Using her new technique, she touched in first place at every turn to claim the gold medal, 0.1 of a second faster than Japan's Hideko Maehata, improving on her previous Olympic record.

Though she successfully defended her national title in 1935, she was controversially omitted from the team for the 1936 Summer Olympics in Berlin, and subsequently retired.

She died in 1971 in Manly, New South Wales at the age of 55 from cancer, and was posthumously inducted into the International Swimming Hall of Fame in 1982.