Walter Reginald Brook Oliver was born in Launceston, Tasmania, on 7 September 1883, the son of Henry Oliver and his wife, Josephine Caroline Stevenson. The family came to New Zealand in March 1896, settling first near Warkworth, but moving to Tauranga in the following year. Here, Oliver continued his education at Tauranga School. He started work in the Customs Department in Wanganui in 1900 and later shifted to Christchurch.
As a boy in Tasmania he had begun to collect shells, and this early interest in natural history was to develop into a lifelong involvement with biology, geology and the history of life on earth. He began to develop these wider interests in Christchurch, where he was influenced by a group of amateur naturalists. From 1905 Oliver collected specimens of plants regularly, sometimes monthly, until his death in 1957.
He was transferred to Timaru in 1907 and here he became involved in planning an expedition to the Kermadec Islands in collaboration with a local naturalist, W. L. Wallace. The expedition landed on Raoul Island on 31 December 1907 and returned to Auckland on 16 November 1908. The breadth of the observations and collections made by Oliver on this expedition is evident in seven major scientific papers he published on the island's geology, vegetation, reptiles, mammals, birds and molluscs. Further collections made by him were studied extensively by other scientists.
Oliver then returned to work in the Customs Department in Christchurch and continued his local botanical collecting trips. He visited the Chatham Islands in December 1909 and Stewart Island in November–December 1910. After being transferred to Auckland in 1912, he concentrated on the local botany, especially of the islands in the Hauraki Gulf. He spent 15 days on Lord Howe Island in November 1913 and later published a paper on the vegetation.
During the First World War he was called up in mid 1917 and saw active service in France in 1918 as a corporal in the Canterbury Infantry Regiment. He was discharged in London at the end of the war and spent two months examining specimens of New Zealand plants in the collections of the Royal Botanic Gardens, Kew, and the British Museum (Natural History). He also made extensive notes on museum techniques, which would serve him well in the future. On his way home in 1919 he made significant collections of plants in Tahiti. Back in New Zealand, he resumed working with the Customs Department and was stationed in Auckland. There, on 7 September 1920, he married Isabella Anne Cardno of Devonport.
That year was also a major turning point in Oliver's career: soon after his marriage he was appointed senior scientific assistant at the Dominion Museum in Wellington. He had now, in his mid 30s, become a professional scientist and faced the need for professional qualifications. Fortunately, he was given the opportunity to acquire them. In 1924 he began part-time studies at Victoria University College, completing a BSc in 1927, and an MSc with first-class honours in 1928. In the same year the director of the Dominion Museum, J. A. Thomson, died, and Oliver was appointed in his place. In 1934 the University of New Zealand granted him a DSc.
By this stage the plans for a new building to house the Dominion Museum and National Art Gallery at the Mount Cook site in Wellington were moving slowly towards fruition. Oliver had expressed disappointment at the final decision to move the museum from its central position in Museum Street just behind Parliament Buildings, but enthusiastically set about meticulous planning for the layout of the new building and the transfer of the collection and exhibits. These were times of economic depression and the museum staff and finances were tightly controlled. Oliver's planning ensured that the massive shift of large, valuable, and often very delicate specimens was carried out without major damage. Actual construction work on the new building did not begin until March 1933 and the transfer of specimens could not start until 1 April 1936. The arrangement of the new exhibitions in the much larger building had to be completed in a few short months as the galleries were to be officially opened to the public on 1 August that year. Priority was given to these public exhibits, with many of the research collections not finally unpacked and rehoused until years later.
Oliver recognised that better display techniques were required and that more time and effort would be needed to achieve the highest standards. He was awarded a Carnegie Corporation travel grant to study developments in museums overseas in 1937–38 and arrived back full of plans, but his hopes were dashed by the onset of the Second World War. The immediate effects were reductions in staff and income. The remaining staff were able to continue some research work, but major developments were impossible. Then, on 8 June 1942, with very little notice, most of the museum and art gallery building was taken over for defence purposes. The public was excluded and all the work rooms, library and reference collections were unceremoniously crammed amongst the exhibits on part of one floor. The building was not reopened to the public for seven years, and Oliver's plans for museum development were therefore in limbo. Together with the remaining small scientific staff, he returned to his research. His period of government service was extended because of wartime exigencies, and he did not retire until March 1947.
After his retirement Oliver continued field work and research, with more extensive efforts in Fiordland in 1949 and on Norfolk Island in 1956. He spent 1948 as relieving director of Canterbury Museum. His wife, Isabella, died suddenly on 1 December 1954, and in Masterton on 24 February 1956 he married Helen Charlotte Laing, a librarian, who had accompanied him on his trip to Norfolk Island.
The outstanding feature of Oliver's scientific work was the eminence he achieved in a wide range of diverse areas. His revision of T. F. Cheeseman's Manual of the New Zealand flora, published in 1925, and his monographs of such plant genera as Metrosideros, Dracophyllum, Coprosma, Coriaria and Aciphylla earned him worldwide recognition as a botanist. His works on birds – particularly his major book, New Zealand birds (1930, with a major revision in 1955), his monograph on the moa (1949) and his paper on bird evolution (1945) – gave him equal status as an ornithologist. His account of the whales and dolphins of New Zealand (1922) remained the standard work on this group for many years. He retained his early interest in molluscs and published several useful revisions. Another early interest was inter-tidal ecology, and his 1923 paper on marine littoral plant and animal communities was a pioneer work of international standing.
Early in his career Oliver had established a technique for filing information obtained from publications and from his own observations in a form from which it could be readily retrieved. 'Oliver's System', perhaps unique in the pre-computer age, gave him access to detailed information on an enormous range of individual plant and animal species and of biological topics. Oliver's very modern ideas for the establishment of an organisation for museums in New Zealand were clearly set out in a booklet published in 1944.
Oliver was active in the affairs of a number of scientific bodies, especially the New Zealand Institute (later the Royal Society of New Zealand): he served on the board of governors (1929–33), on council (1934–56), was president (1952–54), and editor (1949–52). He was elected a fellow in 1927, and received the Hector Memorial Medal and Prize in 1936 and the Hutton Memorial Medal in 1950 (both premier awards for research work). He was also a member of many national and international societies and was honoured by election as an honorary member to numbers of these, including the Swedish Phytogeographical Society, the Royal Australasian Ornithologists' Union, the British Ornithologists' Union, and the Fiji Society. He was president of the Wellington Philosophical Society in 1929–30, the first president of the New Zealand Association of Scientific Workers (later the New Zealand Association of Scientists), chairman of the botany section of the seventh Pacific Science Congress in Auckland in 1949, and of the eighth congress in Manila in 1953, and president of the Royal Society of New Zealand's eighth Science Congress in 1954.
Oliver's apparently slight physique and his modest unassuming manner gave little clue to his capacity for sustained physical and mental endeavour, or to his forceful definitive professional writing. Oliver died in Wellington on 16 May 1957, survived by his wife and three children of his first marriage.