Your New Zealand flax 'Guardsman' is known botanically as Phormium tenax 'Guardsman'.
Phormiums do get fungal infections such as root rot and basal stem rot.* It comes from a pathogen in the soil.* You don't say how long ago you planted them, but if it's recent I would suggest you take them back to the place of purchase for a refund or replacement.* I don't suggest you plant others in the same spot unless you also replace that soil or your new plants will be at risk as well.* From this site:
"Summaries (En, It)
Abstract*** Root and basal rot caused by soil-borne Phytophthora spp., on a flowering plant, Limonium sinensis, and on two foliage crops, Phormium tenax and Viburnum tinus, are reported. On Limonium, the infected plants quickly collapse and show a root and basal rotting. On Phormium, symptoms are a quick collapse of the entire plant, a putrefactive rotting of the roots and an alteration of the base with internal tissues showing a brown-reddish colour."
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.