The culprit is Trionymus diminutus, a 4mm sap-feeding mealybug that breeds relentlessly when the weather is warm and isn't killed by winter frosts. It deposits a white, waxy substance at the base of the sheathed phormium leaves. A mild infestation is not a problem, but in excess the plants will suffer and could even die.
Smudges of white, waxy deposits appear at the base of phormium leaves, deposited by sap-feeding insects. The mealybug can cause loss of vigour in the plant and, in extreme cases, death.
Destroy infested plants. Check new plants carefully to make sure they are not infected.