Thomas Brunner was baptised on 22 August 1821 at Oxford, England, the son of William Brunner, attorney at law and Oxford county coroner, and his wife, Elizabeth Ann Fraser. At about the age of 15 Brunner began five years' service with Thomas Greenshields of Oxford, architect and surveyor. In March 1841 his father signed him on for three years with the New Zealand Company as an 'improver' or apprentice surveyor. On 9 September he arrived at Port Nicholson (Wellington) aboard the Whitby, as a member of the 77-strong advance party for the Nelson settlement; the party reached Nelson Haven on 4 November, and for two years Brunner assisted in the laying out of Nelson's sections and roads.
Nelson lacked extensive lands for pastoral farming. The discovery of the Wairau plain appeared to meet the need, but after the Wairau incident in June 1843 settlers centred their hopes on the southern regions of the province. By August 1843 Brunner was a member of survey parties exploring Nelson's interior, and had joined forces with Kehu, a Ngāti Tūmatakōkiri Māori.
After a fruitless attempt to discover great plains said to exist in the interior, Brunner persisted in his explorations. In February 1846 he and Kehu joined Charles Heaphy and William Fox in a month's exploration of the upper Buller River and its tributaries, which took them as far as the Maruia River, near Murchison. On 17 March Brunner, Kehu and Heaphy left Nelson again and, via Golden Bay, travelled the length of the West Coast as far as Hokitika. On their five month return journey Brunner and Heaphy became the first Europeans to visit the Poutini Ngāi Tahu settlements at Māwhera, Taramakau and Arahura, and to identify Mt Cook as New Zealand's highest mountain.
Brunner began his greatest journey on 11 December 1846, determined to trace the Buller River to the sea; to traverse the West Coast as far south as Milford Sound; and to find a pass across the Southern Alps. Assisted by Kehu and Pikewati and their wives, Brunner spent 14 weeks negotiating the Buller River's gorges, enduring continual storms and floods. The party was forced to eat Brunner's dog to survive.
Brunner regained the Poutini Ngāi Tahu settlements on 1 July 1847 and spent the winter there before continuing south with new Māori companions. By now Brunner could boast (or write) of being able to walk barefoot, and of learning the proper method of cooking and eating fern root. He traversed the south-west coast as far as Paringa before an ankle injury forced him to return north. He regained Māwhera by Christmas 1847.
Early in 1848 he began his return journey to Nelson via the Grey and Inangahua valleys. In the upper Buller Gorge he suffered a stroke, which left him paralysed on one side, but with the aid of Kehu he reached Nelson again in June 1848. Although he did not accomplish all his objectives, Brunner was able to reveal to European settlers the nature of the entire West Coast region including the presence of usable coalfields in the lower Grey valley.
Brunner received an award from the Royal Geographical Society in 1850 as a result of his expedition, but ill health limited his ability to undertake further strenuous journeys. After a brief spell with the Canterbury Association Brunner was appointed surveyor of Crown lands in Nelson in 1851, and Nelson province's chief surveyor, commissioner of public works and commissioner of native reserves in 1856. In 1862 he returned to the West Coast, laying out the towns of Westport and Greymouth.
Stoic and determined as an explorer, Brunner was also a popular member of Nelson society, being regarded as a pious man of the highest integrity. He was made a justice of the peace and was appointed a visiting justice, a deputy sheriff and a member of the local board of health. He married Jane Robson in Nelson on 11 October 1855; they had no children.
Brunner was retired unusually early, in 1869. Though kept on by the provincial government as a consultant surveyor, he was regarded as 'rather impracticable' by his superiors. The inaccurate state of the Nelson survey, revealed by Major H. S. Palmer's investigation in 1875, was partly Brunner's responsibility. Brunner died at Nelson on 22 April 1874. His funeral at Nelson Cathedral was attended by several hundred people, including a large Māori contingent. Kehu was chief mourner. Brunner's memorials are the rocks of the Buller Gorge and the surf-riven coast of South Westland. They continue to bear witness to the greatest single piece of overland exploration in New Zealand's European history.