women four-to-one. Free settlers began arriving in large numbers from 1820, lured by the promise of land grants and free convict labour. Settlement in the island's northwest corner was monopolised by the Van Diemen's Land Company, which sent its first surveyors to the district in 1826. By 1830 one-third of Australia's non-Indigenous population lived in Van Diemen's Land and the island accounted for about half of all land under cultivation and exports. Black War Tensions between Tasmania's Aboriginal and white inhabitants rose, partly driven by increasing competition for kangaroo and other game. Explorer and naval officer John Oxley in 1810 noted the "many atrocious cruelties" inflicted on Aboriginal people by convict bushrangers in the north, which in turn led to black attacks on solitary white hunters. Hostilities increased further with the arrival of 600 colonists from Norfolk Island between 1807 and 1813. They established farms along the River Derwent and east and west of Launceston, occupying ten percent of Van Diemen's Land. By 1824 the colonial population had swelled to 12,600, while the island's sheep population had reached 200,000. The rapid colonisation transformed traditional kangaroo hunting grounds into farms with grazing livestock as well as fences, hedges and stone walls, while police and military patrols were increased to control the convict farm labourers. Violence began to spiral rapidly from the mid-1820s in what became described as the "Black War". Aboriginal inhabitants were driven to desperation by hunger - that included a desire for agricultural produce, as well as feeling anger at the prevalence of abductions of women and girls. New settlers motivated by fear carried out self-defence operations as well as attacks as a means of suppressing the native threat - or even in some cases, exacting revenge. Van Diemen's Land had an enormous gender imbalance, with male colonists outnumbering females six to one in 1822and 16 to one among the convict population. Historian Nicholas Clements has suggested the "voracious appetite" for native women was the most important trigger for the explosion of violence from the late 1820s. From 1825 to 1828 the number of native attacks more than doubled each year, raising panic among settlers. Over the summer of 18261827 clans from the Big River, Oyster Bay and North Midlands nations speared stock-keepers on farms and made it clear that they wanted the settlers and their sheep and cattle to move from their kangaroo hunting grounds. Settlers responded vigorously,