It will be difficult to fell tree due to adjacent structures. Is there a way to kill tree,to prevent future growth & dismantle tree as it dies ? Pohutukawa is New Zealand Christmas tree.
These articles should help:
https://www.gardeningknowhow.com/ornamental/trees/tgen/problem-tree-roots.htm
I made a bed around the bottom of the tree, but, while the shade plants I put there don't die, they don't prosper either.
The best thing you can do for your tree is to apply 2 to 3 inches of hardwood mulch beneath the tree out to the dripline. Trees have feeder roots within a foot of the ground and planting under the tree not only can disturb the roots, but the tree and underplantings are competing for nutrition and water. Neither can thrive.
If you have your heart set on planting under the tree, here are some guidelines to avoid injuring the tree.
https://www.gardeningknowhow.com/ornamental/trees/tgen/planting-garden-beneath-trees.htm