Umlalazi Nature Reserve is directly adjacent to the town of Mtunzini, 120 km north of Durban. It is bounded to the north by the Mlalazi River and its lagoon; to the west by Mtunzini Fish Farm, Zini River Estate and commercial sugarcane and timber lands; to the south by the Mlaba Traditional Authority and Amatikulu Nature Reserve; and to the east by the sea. It is bisected by the Siyaya River, which has its own lagoon. The terrain is flat, ranging in altitude from zero to 30 m a.s.l. The base rock is of recent origin and almost all the reserve consists of Quaternary sands. The climate is humid and subtropical. Rainfall averages 1 350 mm p.a., falling mostly in summer.
Dune forest, both pioneer and mature, occupies much of the reserve, although further inland the diversity of tree species increases. All other vegetation communities in the reserve are hygrophilous. Wet areas beyond the influence of brackish water bear extensive stands of Phragmites reeds. A swamp forest fringes the Siyaya Lagoon, whereas a mangrove forest surrounds the Mlalazi Lagoon. In the upper tidal reaches of the mangrove forest there is a transition to salt marsh. Mudflats bear a sparse cover of plants.
The belts of coastal forest are important wintering grounds for Spotted Ground Thrush Zoothera guttata and provide habitat for Green Twinspot Mandingoa nitidula, Black-throated Wattle-eye Platysteira peltata, Black-bellied Starling Notopholia corrusca and Grey Sunbird Cyanomitra veroxii. Woolly-necked Stork Ciconia episcopus and Crowned Eagle Stephanoaetus coronatus regularly nest in the reserve.
The mangroves in the estuary hold a small wintering population of Mangrove Kingfisher Halcyon senegaloides, which is present from the end of March to early September. The fresh-water section of the river has many backwaters, with overhanging vegetation suitable for White-backed Night Heron Gorsachius leuconotus and African Finfoot Podica senegalensis. Swamp Nightjar Caprimulgus natalensis has been recorded in damp grassland upstream from the lagoon, and Bat Hawk Macheiramphus alcinus in the coastal dune forest. Other raptors include Black-chested Snake Eagle Circaetus pectoralis and Southern Banded Snake Eagle C. fasciolatus. This site is also well known for the small, but stable population of Palm-nut Vulture Gypohierax angolensis, 3–4 pairs of which are resident. Birds may be seen in the stands of Raphia australis palms where they breed, or on the lagoon mudflats where they forage. Adjacent grassland and reedbeds host small populations of breeding Red-headed Quelea Quelea erythrops. Pel's Fishing Owl Scotopelia peli was recorded until the late 1970s or early 1980s but has not been recorded since.
Globally threatened species are Spotted Ground Thrush and Southern Banded Snake Eagle. Regionally threatened species are White-backed Night Heron, African Finfoot, Mangrove Kingfisher and Half-collared Kingfisher. Fairly common biome-restricted species include Kurrichane Thrush Turdus libonyanus, Black-bellied Starling, Mangrove Kingfisher, Gorgeous Bush-Shrike Chlorophoneus viridis and Grey Waxbill Estrilda perreini.
The orchid Didymophlexis verrucosa – unusually, a bark saprophyte – is endemic to the reserve and its immediate surrounds. Two Red Data fish, broadhead sleeper Eliotris melanosoma and bearded eelgoby Taenioides jacksoni, are almost certainly present in the estuary. Eight Red Data reptiles occur, including Nile crocodile Crocodylus niloticus, African python Python sebae and gaboon adder Bitis gabonica. The last was introduced in 1994 as a conservation measure, as Umlalazi could be used as a sanctuary, being almost the only safe and suitable place not yet inhabited by this species. Red Data mammals present include African striped weasel Poecilogale albinucha, blue duiker Philantomba monticola and red duiker Cephalophus natalensis. The critically endangered Pickersgill's reed frog Hyperolius pickersgilli has been recorded in wetland habitats in the reserve.
Facing few significant threats itself, Umlalazi Nature Reserve conserves important coastal dune forest, grassland and swamp and mangrove forests that are under major threat along the rest of the KwaZulu-Natal coastline. The Fairbreeze mine is a controversial operation that will border the reserve and the town of Mtunzini. The footprint of the mine will not destroy natural habitat, only sugarcane farmland and eucalyptus plantations. The main concerns around the mine relate to the hydrological implications it may have on the Mlalazi River estuary and the Siyaya River.
The Mlalazi River and its estuary receive nutrient-enriched waste water from the Mtunzini Fish Farm. So far no major damage has been noted, but the scale of pollution could increase. Five private properties remain as an enclave within the reserve. They are a source of cats and dogs, and although this seems trivial, uncontrolled cats pose a major threat to Spotted Ground Thrush. These birds are habituated and often forage on the ground near dwellings. Invasive alien plants also escape from the gardens into the reserve, with birds contributing to their dispersal. Tourism-related activities such as boating and river tours disturb species in the estuary.
Umlalazi was proclaimed an EKZNW reserve in 1948 and forms part of the Siyaya Coastal Park, which includes Amatikulu Nature Reserve to the south. The IBA should be expanded in the future to include the entire Siyaya Coastal Park. The mangrove swamp is one of the finest remaining examples in South Africa and the most accessible for education purposes. The dune forest is also an excellent example of its kind and is noteworthy for the presence of all phases of dune plant succession in a visibly dynamic process. Raphia palms, first planted in Mtunzini in 1915, have become self-perpetuating and are now established in the reserve, where Palm-nut Vultures feed on the fruits and nest in the trees.