This is a Dracophyllum,one of the plants that gives New Zealand forests - at least to a northerner's eye - a distinctly exotic appearance. Though it doesn't look it, it is a relative of heathers and heaths (Ericaceae), as one of the plants formerly included in a separate, largely southern-hemisphere family, Epacridaceae.
In most parts of the world, if you found one tree busily engaged in trying to choke the life out of another, you would be safe in assuming that the murderer in question was a strangler fig. In New Zealand, though, it is more likely to be a northern rata (Metrosideros robusta), a member of the myrtle family (Myrtaceae). While other ratas (and other Metrosideros spp., like the famous ohia lehua (M. polymorpha) of Hawaii) usually grow from the ground up like respectable trees, the northern rata starts life as an epiphyte high in the canopy. In time, like a strangler fig, its roots reach to the ground, forming a robust pseudotrunk that envelops and, eventually, smothers its host (though there is some argument as to whether the rata actually contributes to the death of the host tree).
The result can be a truly impressive tree, reaching 25 m in height. This one, it seems is well on its way.