Hi bacchus, nice to see another kiwi on here.
Sad circumstances though.
What are you worming your chickens with, how are you administering it, and what dose are you using? People on here recommend Flubenvet (Flubenol) but I haven't been able to find a source of it in NZ.
P.S. I know a couple of experienced chicken-keepers down your way so send me a PM if you're interested and I can put you in touch.