Complex sentence: The building was the first, large-scale all steel-framed "skyscraper" in Australia, known as the Sydney Bank it was headquarters of the Commonwealth Bank of Australia, which then served at the time as the Australian central bank.
Split into simpler sentences: The building was the first, large-scale all steel-framed "skyscraper" in Australia. Known as the "Sydney Bank" within the Commonwealth Bank, it was built to be the headquarters of the Commonwealth Bank of Australia, which served at the time as Australia's central bank.