Complex sentence: Competitive swimming was not well developed in Western Australia: races were held in muddy river pools; hence, in late 1955, O'Halloran moved to the east coast to support his attempt to qualify for the Olympics.
Split into simpler sentences: Competitive swimming was not well developed in Western Australia; races were held in muddy river pools. So in late 1955, O'Halloran moved to the east coast to support his attempt to qualify for the Olympics.