Carex flagellifera 'Frosted Curls'. A very hardy dwarf grass that will tolerate just about any conditions. It grows to about 40cm x 50cm. It is a lovely green colour fading to white ends. It will soften the stronger spikey green plants of the Flax and Yucca/Cabbage tree.
Mondo grass. Both varieties pictured here - Ophiopogon planiscapus 'Black Dragon', the dark red variety, and Ophiopogon japonicus, the green variety. You could use either one, but I think the green variety would look better with the other colours used in the garden.
Possible dwarf shrub - Metrosideros 'Tahiti' - dwarf pohutukawa. Grows about 1m x 1m and has gorgeous red/orange flower and silver/grey foliage with reddish stems. Makes a great contrast to dwarf flaxes and native grasses.
Scleranthus biflorus, New Zealand native moss. A very low growing ground cover, that almost forms a green spongy cushion. If it starts to brown off in the middle, carefully push it down so the roots can re-establish in the ground.