Donald Frederick Wünsch was born on 16 January 1887 at Chorlton on Medlock, Manchester, England, the eldest son of Mary Sandys and her husband, Edward Wünsch, a shipping merchant. After going to school in Manchester, he studied natural science at New College, University of Oxford (1905–9), graduating with an MA in chemistry and distinction in mineralogy. From there he went to McGill University, Montreal, Canada, where he graduated with a BSc in 1911. Later that year he became assistant engineer to the superintendent of West African Mines and worked for Ropp Tin in northern Nigeria; by 1914 he was manager of West African Mines.
After the outbreak of the First World War Wünsch returned to Britain and worked as a shift manager at a munitions factory in Northwich, Cheshire. There he met Thekla Josephine (Josie) Bowker, and they were married on 8 June 1916 at Llangian, Caernarvonshire. In 1920 he returned to Nigeria for 18 months as general manager of Dua (Nigeria) Tinfields, leaving his wife and their only child, a daughter, in England. On his return he was works manager for J. B. & W. R. Sharp, manufacturers of aniline dyes at Edenfield, Lancashire. He then bought two acres of land in Cambridgeshire and became an orchardist; he was to retain a love of gardening all his life. In 1923 he was a founding member of the Institution of Chemical Engineers, London.
In 1928 Donald Sandys Wunsch, as he now preferred to be known, was recruited by Leonard Harding to manage the factory of the New Zealand Sugar of Milk and Casein Company at Edendale, Southland. At the time the company was near bankruptcy after two disastrous fires and mismanagement. On his arrival in New Zealand in 1929, Sandys Wunsch was initially tempted to return to Britain, but he felt an empathy with the staff who worked so willingly seven days a week during the season. Assisted by Maurice Turner and Jack Bayley, he had the factory scrubbed from top to bottom and changed its processing system to the 'English' method. He combined his wide chemical and engineering experience with considerable financial acumen: he sent sellers overseas to secure orders, assured Truby King of a steady supply of lactose for the baby product Karilac, and also developed a stock food, Vilac.
In 1934 the company changed its name to Dairy Products. A progressive employer, Sandys Wunsch initiated worker participation in management by forming a works council. As managing director he reported directly to employees each month on production, sales and future plans. Demand increased during the Second World War because lactose was used in the production of penicillin. By 1950 the factory's staff had increased to 68 and it had become the largest producer of lactose in the world.
Sandys Wunsch was convinced that the lactose industry needed not chemists but chemical engineers, and as professional qualifications were not attainable in New Zealand he trained several himself. He also encouraged the founding of a chemical engineering department at Canterbury University College in 1944. Although he is widely regarded as the leading figure in the development of the New Zealand lactose industry, Sandys Wunsch also made a major, but less well-known, contribution to agricultural science.
In 1933 he undertook pioneering research on the causes of 'bush sickness', a disease that afflicted livestock in various parts of New Zealand, particularly at Morton Mains in Southland, in parts of South Canterbury, and on the vast tracts of pumice land in the central North Island. Sandys Wunsch suggested that a cobalt deficiency could be the answer. Unfortunately, his findings were largely ignored, except by the farmers at Morton Mains who had supplied the sheep he worked with. Two years elapsed before other scientists confirmed that the prevention of bush sickness lay in the use of cobalt.
Sandys Wunsch was a member of the Council of Scientific and Industrial Research from 1943 to 1958 and was its chairman from 1955 to 1958. He became a fellow of the New Zealand Institute of Chemistry in 1944, and in 1957 was made an OBE for his services to science. In 1944 he had helped to select the site of the new Dairy Products factory at Kapuni, Taranaki. After supervising its construction, he retired to New Plymouth in 1950. He continued to act as an advisory and consulting director and was also a director of McKechnie Brothers New Zealand Limited.
A tall, thin man with a shock of red hair, a bushy red beard and very blue, intelligent eyes, Donald Sandys Wunsch wore small glasses, English tweed knickerbockers, and was rarely seen without a pipe in his mouth. When the artist Peter McIntyre sketched him in later life, his receding hair was white, his beard was a neat goatee, but his eyes were still bright, and the rimless glasses and pipe remained. His hobbies included fishing and shooting, and he was a life member of the Edendale Sports Society. His main interest in retirement was gardening and he created two beautiful gardens at his New Plymouth homes. Sandys Wunsch died suddenly in his garden on 23 August 1973, survived by his daughter. His wife, Josie, had died in 1966.