In the initial times, Australia was a penal colony of the British Empire, and many Irish Catholics were sent there, but they were deprived of any spiritual sustenance.
In accordance with St. Columbans Mission Society , “The infant church in Australia had a special reason for turning to Mary. So, no priests were sent to the colony in its early days and Mass was not allowed until 1820. In those days, it was mostly the Rosary that kept the faith alive.”
By 1820 the Catholic Church was given some freedom, and in 1821 a cathedral in Sydney was dedicated in honor of the Virgin Mary.
Prior to that, Pope Pius VII was detained and was a prisoner of Napoleon. He was released in the year 1814 and he made his way back to Rome. So, in 1815 Pope Pius VII “instituted the feast of Our Lady, Help of Christians, for the Papal States to be celebrated on 24 May, the anniversary of his first return.”
As a result of that, Australia became the first nation to choose “St. Mary, Help of Christians” as their principal patroness in the year 1844. As May 24 then became the “patronal feast” of Australia and also remains a solemnity, though it can also be celebrated on the nearest Sunday.
The cathedral in Sydney has as well, long adopted the title, Immaculate Mother of God, Help of Christians.