It may surprise some people that Flax broadly fits in to two different categories (the coloured versions are simply offshoots of this).
In simple terms Swamp Flax (Harakeke) grows in wet areas and Mountain Flax in drier areas. A simple way to tell the two apart is the upright nature of the flowers on Swamp Flax and drooping/side shooting habit of flowers on Mountain Flax.
Growing to around 1.5m wide/high mountain flax is great for mass planting in landscape scenarios and around things like cattle yards and cowsheds. It provides good cover and the flowers are welcome food for Tūī and the like in early spring.
When planted as a part of a revegetation project it's important to plant them alongside other low growing species otherwise they will quickly be shaded out and will perish.