Seaweeds or marine algae are relatively simple plants. The smallest consist of only a single cell, or the plant body (thallus) may be a simple filament, a broad sheet, a frond with flat or cylindrical divisions, or a close or crumpled crust on rock, shell, or other substratum. Even the largest absorb food over the whole surface, the rootlike portion being an attaching holdfast only. Some, for instance Corallina, that are hard and brittle with limy salts extracted from the sea, are related to the nullipores of coral reefs. In others, for example Carpophyllum, hollow vesicles buoy up the thallus towards the light; in many, watery or mucilaginous contents reduce the risk of drying out at low tide. Reproduction is by single cells produced over wide areas of the surface, as in bull kelp, in restricted patches, as in Stenogramme, or on special branchlets, as in Carpophyllum. Classification is based on reproductive pattern, but three large groups are distinguished by colour. All contain chlorophyll clearly seen in the green Chlorophyta but obscured by brown pigments in the Phaeophyta and by red of different shades in the Rhodophyta. Recorded numbers of species in New Zealand are: green, 205; brown, 127; and red, 389. About 40 per cent are endemic and at least a hundred species are shared with Australia.

Microscopic seaweeds floating in the surface layers of the sea form the phytoplankton which traps the energy of the sun and is the basic food of all sea animals. In New Zealand waters 198 species have been recorded, of which 125 are Bacillariophyceae (diatoms) and 63 Dinophyceae. Phytoplankters sometimes occur in large discoloured patches and give the brown tint to blown foam on west-coast beaches. Their abundance varies greatly in different current zones.

Larger seaweeds are mostly firmly attached, each where conditions suit it best. The bladder kelp, which reaches a length of 100 ft or more, is common about Cook and Foveaux Straits and the east side of the South Island, but is not known to grow on the west coast of the South Island or north of Castlepoint. It has a south circumpolar distribution, but extends up the west coast of America and as far north as California, and a closely related species grows on the west coast of South Africa. Marginariella, an endemic genus of two species, and the strongly acid Desmarestia, whose relatives are widespread in temperate regions of both hemispheres, and a number of red seaweeds are likewise restricted to southern New Zealand. Carpophyllum plumosum, a very common large brown weed of the North Island east coast, is unknown in the South Island, though it grows at Chatham Islands.

Durvillea antarctica, the bull kelp, is a species on rough coasts, and Hormosira banksii, the necklace seaweed, grows only where it is sheltered from heavy seas. Upper intertidal levels are occupied by species such as Porphyra columbina, the karengo, that tolerate long exposure to air, light, and rain. The falling tide uncovers plants that are successively less tolerant, for example, Hormosira and the green velvety Codiums. The green grape seaweed, Caulerpa sedoides, is left dry only at very low tides. Many weeds, both tough, like Zonaria, or delicate in texture, like Laingia, are sublittoral, growing below the level of low spring tides. Karengo is seasonal, appearing in winter as narrow silky ribbons which, by early spring, have grown into broad blackish-purple sheets a foot or so long; by summer all has worn off except the holdfast with a ragged yellowish green frill. Desmarestia appears on low-tide rocks in early winter as a delicate branching frond fringed with soft hairs; by summer it is 6 ft long, dark brown, hairless, and with a stout midrib in each segment. Scytosiphon, like a string of miniature brown sausages, and Adenocystis, the sea bomb, are also annual seaweeds.