Nectar when they can get it, but otherwise insects and berries. I grow a couple of Tree Lucerne to encourage the Tui and Bellbird in winter when few other trees are flowering. Many of the Eucalypts...
Nectar is the normal diet but fruit and insects are often eaten, and occasionally pollen and seeds. Particularly popular is the New Zealand flax. Tui are the main pollinators of flax, kowhai, kaka...