Species of Castanopsis and Lithocarpus are the predominant trees in the lower montane forests, which extend from approximately 1000 to 2,000 meters elevation.

The mountains are home to the most extensive unlogged Dacrydium forests in the Southern Hemisphere.

[1] The climate in the region is typical of tropical rainforests, with high temperatures, humidity, and rainfall all year round.

[2] The temperatures in the Cromwell Mountains hover around 25°C (77°F) throughout the year, except for higher elevations where it can get cooler.

The Southeast Trade Winds bring moist air from the Pacific Ocean, leading to high levels of precipitation.