I will go to the Vets and ask them what spray works best. I also noticed this morning that she is very skinny. So far none of my other chickens have gotten sick. Do you know if his could spread to my ducks and geese or is it only a chicken disease?
If you have vet care, that is always best.
Ask if they can run a fecal float to see if worms and coccidiosis are part of your problem in addition to the bugs (lice).
Can you post some photos of your hens and their poop? What do you feed including treats?
If you can't get help from the vet, then I would treat for the lice. You are in AU, so you should be able to find Pestene or another louse powder or miticide. Ask at your local feed store or pet shop.
For deworming, you can use Levamisole.
You mention having a 5 month old that died. Do you have others this age and are they doing ok? How long have you had them and how long have they been with the flock that is not well?
I would be tempted to treat the sick hen for Coccidiosis as well. Look for Coccivet or Harker's Coxoid both are Amprolium. Dosing instructions are on the bottles for those.
This is all IF you can't see the vet, hopefully you can and they can determine what treatment(s) are best given. What I have suggested will cover the most common parasitic problems chickens can have, but if you are dealing with a disease like Mareks or some other illness/condition, none of these may be effective.