Experts and BHG readers answer.
Can dying Platts Black New Zeland Flax be saved?
The roly polys are not the cause, they are just there to eat the decaying plant matter. Sounds like you have a rot problem and these are usually a result of too much watering and/or poor drainage. Cut back on irrigation and improve drainage if you can. If not, maybe this isn't the best site for a NZ flax. They like it hot and dry(ish).