Complex sentence: World War II baby boom and the immigration of many young people who then had children in Australia, with a plateau of rates of over 13.0 persons per 1,000 population for every year from 1946 to 1962.
Split into simpler sentences: World War II baby boom and the immigration of many young people who then had children in Australia. A rate plateau of over 13.0 persons per 1,000 population occurred for every year from 1946 to 1962.