A photo would help to be sure, but could it be Phormium mealybug? This makes dense white deposits, usually at the base of the leaves, particularly where the leaves clump together.

Gently brush off the white fuzz, and there might be little gray or pinkish "armadillos" underneath--those would be the mealybugs. A severe infestation would make the plants wilt, as you describe.

There's a mealy bug killer in the garden centres and it works - it saved my plant this year.