Drawn by South Africa's healthy climate he settled in Port Elizabeth in 1895 to practice medicine and improve his English. He was licensed to practice in the Cape Colony on 21 October 1895. In 1898 he moved to Bothaville and applied for registration as a medical practitioner in the Orange Free State. However, he soon moved to Hoopstad, where he became district surgeon in 1899. He married Auguste Louise Agustine Grell (1869-1964) in 1900 and they had a son. Around this time his interest in the fauna of the Karoo led him to settle in Willowmore, where he remained for the rest of his life. He became a British subject in 1909.

Brauns collected insects whenever his medical practice permitted during his entire life in South Africa and built up a remarkable collection, particularly of Hymenoptera. He was, however, not merely a collector but the outstanding student of South African hymenoptera of his time, interested in their taxonomy, life histories, and symbiosis among ants. Most of his collecting was done in the Karoo, but he undertook field trips to other parts of the Cape, the Free State and Transvaal. His wife also collected extensively and his son Friedrich later assisted. As early as 1897 Brauns donated some Hymenoptera and Coleoptera to the South African Museum, Cape Town, and followed this up with donations of rare forms of the same two orders in 1900, Coleoptera of the Free State in 1901, more Hymenoptera in 1904 and 1905, as well as some other invertebrates (1901), including a new crustacean (1902). A collection of over 200 identified Hymenoptera was also presented to the Albany Museum, Grahamstown, in 1906, followed by more specimens in 1910. He identified Hymenoptera for that museum in 1912.

In 1925 his collection of Hymenoptera, which was particularly complete with regard to wasps, was inspected in Willomore by C.J. Swierstra* of the Transvaal Museum. Upon the latter's recommendation the museum bought the collection for the then princely sum of £1500. It comprised some 70 000 specimens, representing over 10 000 species.

Brauns was also an authority on Karoo succulents and donated plants to Albany Museum in 1911. The genus Braunsia and the species Argyroderma braunsii and Conophytum braunsii were named in his honour. His interests extended to the geology and palaeontology of the Karoo and he made some notable fossil discoveries.

Hans Brauns produced some 38 scientific publications from about 1889 onwards, several of his earlier papers dealing with the insect fauna of Mecklenburg. However, most of his papers dealt with the Hymenoptera of southern Africa, though many were written in German and appeared in European journals. His first contribution on the fauna of South Africa was a paper, "Zur Kenntnis der südafrikanischen Hymeneopteren" in the Annalen des k.k. Naturhistorischen Hofmuseum in 1898, while his last paper appeared in the year after his death. Papers published in South Africa included "Notes and synonymy of Hymenoptera in the collection of the Transvaal Museum" (Annals of the Transvaal Museum, 1917, Vol. 5(4), pp. 238-245); "The Ethiopian Cerceris species" (Ibid, 1924, Vol. 11(4), pp. 268-); "New genera and species of South African myrmecophilous and termitophilous beetles" (South African Journal of Natural History, 1925); and "A contribution to the knowledge of the genus Allodape", (Annals of the South African Museum, 1926, Vol. 23, Part 3). He also described a new species of myrmecophilous beetle from Southern Rhodesia (now Zimbabwe) in the Proceedings of the Rhodesia Scientific Association (1913-1914, Vol. 13).

Brauns was a member of several foreign scientific societies. He joined the South African Philosophical Society in 1897 and remained a member when it became the Royal Society of South Africa in 1908. He was also a member of the South African Biological Society from 1919 or earlier. In 1927 the University of Stellenbosch awarded him the honorary degree Doctor of Science (DSc) in recognition of his contributions to entomology.