The flora of Westland Tai Poutini differs with altitude. The park's coastline is home to several salt marshes around the lagoons as well as sand dune communities on longer beaches. Flax (Linum usitassimum) and conifers are very common on the coast and in parts of the lowland forest. Beside them, endemic fern species and kamahi-like shrubs are abundant. Examples are native fuchsia, several coprosma species, mosses, orchids and the threatened Carmichaelia juncea.
The rainforest consists of southern beeches (Nothofagus) and is dominated by podocarp species such as the rimu (Dacrydium cupressinum) directed towards the hillcountry, the kahikatea (Dacrycarpus dacrydiodes) found near wetlands as well as the common Southern rata (Metrosideros umbellata). Other noteworthy species include the Elaeocarpus dentatus, Rapanea salicina, and Piper excelsum.
Reaching higher altitudes, forests give space to cold-tolerant shrubs, tussock lands and herb fields starting at about 900 m. Above nearly 1500 m, only lichens may outlive these extreme weather conditions.