Long Island is one of a number of small, forested islands of the Hawkesbury River. It is situated about 50km north of Sydney, across Sandbrook Inlet from Brooklyn, Others include Dangar Island, Spectacle Island, Milson Island, Peat Island and Lion Island. The area has been inhabited for thousands of years by the Guringai people, who left their mark on the land with hundreds of rock engravings, stone sharpening sites, cave paintings and shellfish middens. The first European to see the area was Governor Arthur Phillip, who explored the lower river by small boat in March 1788. At first the local people were friendly towards him, but when he returned a year later, they would not come into contact with him. By 1790 over half the Guringai had succumbed to the smallpox the British had brought with them.
There is some debate about how Long Island was named. Some have claimed the name comes from the time the nearby railway bridge was called 'Brooklyn Bridge' (it was constructed by the Union Bridge Company of Brooklyn, New York, hence the reference to America). Others have pointed out that since it is indeed a long and thin island, some 2 km long by 300m wide, the name may well have preceded this. Whatever the truth, the island is now joined to Brooklyn, and Hornsby Shire, by a railway causeway across Sandbrook Inlet, and to Cogra Point on the northern shore of the Hawkesbury River by the railway bridge. The railway passes through a tunnel at the eastern end of the island, where there are also a few houses with good views of nearby Dangar Island. Like Spectacle Island and Lion Island, Long Island is a nature reserve (73ha, notified in 1972), which means it is illegal to visit without permission. The New South Wales Parks and Wildlife Service runs very infrequent guided tours of the island, for those who are interested in its flora, fauna, geology and Aboriginal history. One of the best views of Long Island is from Lloyd's Trig in nearby Muogamarra Nature Reserve. From this vantage point it is possible to look down on the length of the whole island with the mouth of the Hawkesbury River and Broken Bay in the far distance.
Long Island was added to the Australian National Heritage List in December 2006.