The ecoregion includes the mountain rain forests on the islands of New Britain and New Ireland, which lie northeast of New Guinea.

[2][3][4] The ecoregion includes the montane rain forests on New Britain and New Ireland, two large islands in the Bismarck Archipelago lying northeast of New Guinea.

Temperature generally decreases with altitude, humidity increases, and fog and clouds are common.

Above 1,000 metres (3,300 ft) elevation, species composition and forest structure change.

[5] Common trees in the montane forests include Castanopsis acuminatissima, Nothofagus starkenborghiorum, and the conifer Agathis robusta subsp.

Castanopsis acuminatissima is predominant in montane forests of New Britain and on New Guinea, but mostly absent from New Ireland.