Slide showing a photograph of St. Mary's Cathedral, which is is the cathedral church of the Roman Catholic Archdiocese of Sydney and the seat of the Archbishop of Sydney. The cathedral was built in the second half of the 1800s and is largest church in Australia, though not the tallest.. Boswell Collection. Slide number 28. Sydney is the most populous city in Australia and the state capital of New South Wales. It is located on Australia's south-east coast of the Tasman Sea.