Augustus Hamilton was born at Poole, in Dorsetshire, England, on 1 March 1853, the son of a doctor, Augustus Priestley Hamilton, and his wife, Mary Eleanor Tebbott. He was educated at Dorset county school and Epsom medical college, but did not complete the degree course. In 1875 Augustus emigrated to New Zealand with his parents on the Collingwood. His father was the ship's doctor and Augustus acted as his assistant. During the voyage there was an outbreak of typhoid and 20 passengers died; Augustus and his father also fell ill. The stricken ship arrived at Wellington in July 1875.
Augustus Hamilton taught for short periods in Wellington primary schools, and for a few months in 1877 at Okarito on the west coast of the South Island. He joined the Wellington Philosophical Society in 1876, and for the rest of his life retained membership of one or other of the regional branches of the New Zealand Institute. From 1878 to 1890 he lived and taught in Petane (Bay View), Hawke's Bay. He married Hope Ellen McKain at Napier on 22 September 1882. The couple were to have two children: a daughter, Pearl, and a son, Harold.
During this period Hamilton took an active part in the business of the Hawke's Bay Philosophical Institute, exhibiting items of interest at meetings, serving as honorary secretary (1884–90), and establishing the institute's museum. He came to know such distinguished scientists as William Colenso and Henry Hill, and began to form his collection of Maori artefacts. While living in Hawke's Bay he published a number of scientific papers, mostly recording the occurrence of specific birds and fishes in the area, but more substantial works dealt with fossilised shells and moa bones from Te Aute.
In 1890 the family shifted to Dunedin following Hamilton's appointment as registrar of the University of Otago. During the next 14 years he published nearly 30 scientific papers, many of considerable importance. He compiled a series of comprehensive bibliographies on works dealing with mosses, ferns, Bryozoa, fishes, fishing and moa. However, his most significant activity was in the field of ethnology. As well as publishing a bibliography of literature about the Maori people, he began writing the work for which he is best known, The art and workmanship of the Maori race in New Zealand. Published in parts between 1896 and 1900 and popularly known as 'Hamilton's Maori Art', it soon became a collector's piece. This was fitting, as Hamilton based his observations on a magnificent collection of Maori artefacts, consisting of material he had gathered personally and augmented by purchase and exchange.
The passing of the Maori Antiquities Act 1901 aroused a good deal of interest, within and without Parliament, in the preservation of Maori relics. When a replacement for Sir James Hector as director of the Colonial (later Dominion) Museum was being considered, the need to give more emphasis to the preservation of Maori material was recognised. Hamilton was an obvious choice for the position. When he was appointed in December 1903, he was directed to give special attention to building up a representative series of specimens of Maori art and workmanship. He deposited his own large private collection in the museum and began actively adding to the museum collections. His own standing and the interest he aroused attracted the donation of several important private collections. Hamilton's museum interests were wide: by 1906 he reported that in addition to the establishment of collections relating to Maori history, art and culture the natural history collections of shells, birds and plants were being reorganised. He continued to publish bibliographies and other studies, including two museum bulletins.
Hamilton faced three major difficulties during his 10 years as director. First, the wooden museum building erected in 1865 was by this time too small and in bad repair. He spent much time seeking government approval for extensions on the existing site or a new building elsewhere, to provide secure housing for the irreplaceable collections. On every side his plans were thwarted. His achievement in improving the museum collections has tended to be overshadowed by this lack of success, through no fault of his own, in improving the building. Second, the fossil collections originally amassed by the joint Colonial Museum and Geological Survey under Hector had been legally transferred to the Mines Department, but were still housed in the Colonial Museum. These collections remained a physical and moral responsibility for Hamilton and his successors for several decades. Third, towards the end of his life Hamilton took part in lengthy discussions on the possible establishment of an editorial board for all government publications, and a board of trustees to control the museum. These again were matters that were not to be resolved in his time.
In view of such uncertainties Hamilton decided not to appoint new staff except in a temporary capacity. Amy Castle joined the staff in 1907 to care for the collection of insects – the first entomologist in a New Zealand museum and the first woman appointed in a professional role. She held the position until 1931. Elsdon Best, who was to publish many volumes of work on the Maori, was employed as a 'temporary clerical assistant' from 1910 until his death in 1931.
Throughout his directorship Hamilton carried an extremely heavy workload. Possibly as a consequence of this, in 1910 and 1912 he suffered from some form of 'heart trouble'. Then on 12 October 1913, while working on old church records, he suddenly took ill and died in Russell, in the Bay of Islands. He was survived by his wife and children.
During his time at the museum Hamilton had been deeply involved in the affairs of the New Zealand Institute, serving as honorary editor (1903–8), honorary librarian (1903–13) and president (1909–10). On his death members subscribed to a memorial to be erected on his grave at Russell. Enough money was left over to establish the Hamilton Memorial Prize, a modest award given to recognise especially meritorious publications by young scientists.
Hamilton was interested in a very wide range of subjects: botany, entomology, zoology, geology, archaeology, ethnology and history. He was also a keen photographer and collected stamps, coins and tokens. In writing about his many interests, Hamilton concentrated on recording and systematic listing, to make his findings more accessible for future workers. This scholarly and generous approach to the sharing of information was appreciated by ethnologists such as H. D. Skinner and W. J. Phillipps, who were associated with Hamilton. They remembered most of all his kindliness, his enthusiasm and his willingness to pass on his knowledge.