Horatio Hartley was born in the United States, probably in 1826. His companion Christopher Reilly was born possibly in Dublin, Ireland, where he may have attended Dublin University; they arrived together in Otago in 1862. Their Californian mining experience, and their conviction that the Victorian miners then congregated on the Tuapeka goldfield had not begun to explore the sources of Otago's gold, led them to prospect the Clutha River and its tributaries. Aided by an exceptionally severe winter during 1862, and thus low river levels, they made their major discovery on a beach of the Clutha River, just below its confluence with the Kawarau River. In August 1862 they deposited 87 pounds of gold in Dunedin and, in return for a provincial government reward of £2,000, disclosed the whereabouts of their discovery. On 23 September 1862 the Dunstan goldfield was proclaimed, the selection of this name in preference to Hartley's being largely in deference to Reilly, who was 'jealous of the pre-eminence' accorded Hartley as the discoverer.
After the discovery, Hartley visited and assessed the Coromandel goldfield on behalf of Otago miners, returning to Dunedin in February 1863. On 18 March he left Queenstown for the West Coast via the Dart River, reaching the Awarua River before shortage of supplies compelled his return to Queenstown. His effort, nevertheless, was described at the time as 'a feat of foot which few men could do'. Thereafter, Hartley's movements are uncertain, although he appears to have led a prospecting party from Melbourne to New Guinea in the late 1860s or early 1870s. He died in San Francisco in 1903, leaving US$2,000 to his widow, Mary Anne Hartley, conditional upon her not remarrying, and US$50,000 to the Olympia school district. His widow contested the will on the grounds that Hartley was insane.
Of Hartley's Irish partner, Christopher Reilly, even less is known. After the Clutha discovery, he set out to demonstrate the value of the river as 'Nature's highway to the Dunstan' by leading an expedition of 18 men and 2 lifeboats in October–November 1862. This was part of a larger project to establish a port at the mouth of the Clutha River. It was a remarkable feat, described at the time as 'not less meritorious than that of his discovery of the field'. Although he originally claimed success, Reilly later conceded that the expedition had merely proved that the river was 'wholly unnavigable'. In November 1862 he asked the Otago provincial government to appoint a select committee to consider an award of compensation, the expedition having cost some £615. He had moreover forgone mining on the Dunstan goldfield, 'where he then had an extensive payable claim'. Reilly failed to appear before the committee, and had already left New Zealand when the committee declined to recommend any compensation. In January 1863 he appears in Tasmania, where the government had offered him a reward for the discovery of a payable goldfield. Reilly rejected the terms offered and reappeared in Adelaide. Thereafter his movements and exploits are unknown. He is said to have died in Dublin in 1887.
The disclosure of the finds precipitated a major rush into the interior of Otago. Initially there was considerable dissatisfaction expressed over the apparently limited area of auriferous ground, the scarcity and cost of fuel and supplies, and, indeed, over the value of the field. In the end, Hartley and Reilly's discovery prompted a reassessment of the character of Otago's gold deposits and the introduction and development of new mining technology, and led to a succession of discoveries and rushes which by the end of 1864 had embraced nearly the whole of Central Otago.