behind is the Opera house, Sydney Harbour !
Prior to European settlement, the island had the Eora name Mat-te-wan-ye (sometimes Mallee’wonya). After the First Fleet arrived in 1788, Governor Phillip and his Advocate-General used the name Rock Island. In 1788, a convict named Thomas Hill was sentenced to a week on bread and water in irons there, after which the island came to be known as Pinchgut. Once a 15 metre (49 ft) high sandstone rock, the island was flattened as prisoners under the command of Captain George Barney, the civil engineer for the colony, quarried it for sandstone to construct nearby Circular Quay.
By 1796 the government had installed a gibbet on Pinchgut. The first convict to be hanged from the gibbet may have been Francis Morgan. In 1793, the British transported him to New South Wales for life as punishment for a murder. The authorities in NSW executed Morgan for bashing a man to death in Sydney on 18 October 1796.
In 1839, two American warships entered the harbour at night and circled Pinchgut Island. Concern with the threat of foreign attack caused the government to review the harbour’s inner defences. Barney, who had earlier reported that Sydney’s defences were inadequate, recommended that the government establish a fort on Pinchgut Island to help protect Sydney Harbour from attack by foreign vessels. Fortification of the island began in 1841 but was not completed. Construction resumed in 1855 because of fear of a Russian naval attack during the Crimean War, and was completed on 14 November 1857. The newly-built fort then took its current name from Sir William Thomas Denison, the Governor of New South Wales from 1855 to 1861.