The SS Rotomahana, built at Dumbarton (Scotland) by William Denny in 1879 for the Union Steamship Company of New Zealand is credited with being the very first large merchant vessel to be built in mild steel and the very first steel ocean going vessel. (Dear and Kemp, 2005:521). The Rotomahana was launched on 5 June 1879, leaving London on 5 August on its delivery voyage, and arriving in Melbourne on 22 September, before continuing on to Port Chambers, where the steamer arrived on 1 October, It had accommodation for 140 first class, 80 second class and 80 third class passengers.
On 9 October 1879 Rotomahana left Wellington on her first voyage to Sydney, calling at Napier, Gisborne, Auckland and Russell en route. The steamer remained on this route for the next fifteen years, subsequently taking on the Bass Strait ferry service between Melbourne and Launceston in 1894 for several months before taking on coastal serviced around New Zealand. In 1897 it began service on the Wellington to Lyttleton ferry run, where it remained for several years. After twenty years of service between the North and South Islands, in May 1907 Rotomahana again took up the route between Melbourne and Tasmania. Today the SS Rotomahana, lying in 38 meters of water in Victoria's Port Phillip is a popular wreck dive, visited by hundreds of divers a year.