I have rabbits in my garden, but I haven't tried to grow Phormium (New Zealand flax). My usual answer to protecting individual plants is to make a cylinder of chicken wire, flare out the end and put this round the plant I want to protect, burying the flared out end with a bit of soil or dug out rocks to stop the rabbits knocking it out of the way.

Many plants will survive the rabbit onslaught once they've got established, the bark is toughened up and there are enough branches out of reach of the bunnies.

A one metre length of chicken wire will make a cylinder about a foot in diameter, but I'd go for wider than that if you want your Phormium to reach any size.

According to info I've just looked at, Phormiums are very rarely touched by either rabbits or deer. Not surprising for rabbits, you can make strong baskets out of phormium leaves, probably really tough to chew.