It is an evergreen shrub favouring both bogs and dry stony ground, seldom growing to more than 3.5 m (11 ft) high.

It grows up to 4.8 m (15.7 ft) tall and has a short trunk that Kirk noted "rarely exceeds" 1 foot in diameter,[4] and more commonly has a thickness between 30–40 cm (12–16 in) at breast height.

[6] During the flowering months of October to December, small male cones, 3–5 mm in length, are brown to red at the end of the pine's scale-like leaves.

The fruit of mountain pine consists of a dark brown, black-brown to purple-brown seed in a fleshy, waxy white cup.

Bog pines are easily recognized when fruiting by the waxy white (very slightly yellowish) arils subtending the seed.

Vegetatively compared with other species of Halocarpus, mountain pines have growth habits of smaller multibranched shrubs to small trees, weak keel-shaped leaves, and more slender, initially quadrangular branchlets.

[4][8] Harry Allan disputed the status of these varieties in his 1961 Flora of New Zealand (Vol 1), suggesting instead they were the result of hybridisation.

[10] The specific epithet bidwillii is in honor of John Carne Bidwill (1815–1853) who was an Australian botanist born in England and became the first director of the Royal Botanic Gardens, Sydney.

[4] Three separate studies using DNA sequencing, one in the year 2000, and two in 2002, have confirmed that the genus Halocarpus is monophyletic, as they all descend from a common ancestor, though the wider clade it is part of was found to be unresolved.

A study in 2002 from the journal of Plant Systematics and Evolution found a relationship in which H. kirkii and H. biformis form a clade, to which H. bidwillii is sister.

Halocarpus bidwillii is endemic to New Zealand and grow from Coromandel to the extreme south; as the latitude increases, they are found at lower altitudes.

[16] In the North Island, it can be found in Taupo county near Rotoaira[4] and in the central volcanic plateau and Kaingaroa plains.

[17] In the North Island, mountain pine are found exclusively in montane to alpine habitats and usually between 600 and 1500 m elevation.

Mountain pine can grow in both bog environments and in dry stony ground, with mountain pine growing extremely well in the Te Anau stony ground environment[18] and just as effectively in wetland margins, frost flats, and riverbeds.

Briefly, the 4 main categories of insects that prey on mountain pine are: beetles, sucking bugs, caterpillars, and mites.

[21] More specific, scale insects, Eriococcus dacrydii, live on the stems and leaf scales of the Halocarpus species,[22] and even more specific, (Dugdale, 1996) found a species of conifer associated moth that uses the mountain pine as its host plant, appropriately named Chrysorthenches halocarpi.

[18] Around this same time, ovules grow at the tips of the branches and once fertilized by the pollen they develop a white aril at the base.

Kirk commented on the "attractive character" of mountain pine, citing its symmetrical growth, and suggested that it could become an ornamental plant.