Welcome to BYC!
Duck do get mites and lice. If they do have lice you will need to treat them *and* their coop. To treat the coop I use a permethrin spray. Can you get a permethrin spray in AU?
Ducks have so much down that it makes using a spray or dust on them difficult, so the easiest way to treat ducks for mites and lice is with ivermectin. One dose of ivermectin and all mites and lice should be dead in 24-72 hours. Here in the US we can buy:
- Injectable for cattle which we give orally
- Pour on for cattle
- Oral paste for horses
- Oral for sheep and goats
What ivermectin products can you get in AU? Let me know what you can get and I will calculate a dose for you.
Do people that show ducks bathe them? If so, another option would be to use a dog or cat flea shampoo. If you decide to do that let me know what what you want to use and I will check to see if it is safe for birds.