Hi @Sia1112,
A warm welcome to the Bunnings Workshop community and thank you for your question.
They aren't looking great, but I don't feel they are beyond saving.
The white stuff on the leaves looks to be powdery mildew. I'd suggest using a damp cloth to wipe it off of the affected leaves and then spray them with OCP 750ml Ready To Use Eco-Fungicide.
If there is an obvious insect infestation, I can recommend steps, but without seeing the specific insects it would be difficult. If you can get some photos of the culprits, I'd be happy to advise what can be done.
The main thing we should focus on after managing the mildew and insects is improving the overall health of the trees.
Apply a controlled-release fertiliser such as this Scotts Osmocote All Purpose Controlled Release Fertiliser following the manufacturer's guidance for application rates.
Water them consistently to ensure they are growing to the best of their ability. You'll want to water them deeply around once a week. Check the soil is not still damp before watering again as overwatering can cause just as many issues as underwatering.
Check out How To Grow And Care For A Lilly Pilly and How To Maintain Healthy Lilly Pillies for some guidance.
Allow me to tag @Noelle and @mich1972 for their thoughts.
Let me know if you have any further questions.
Jacob
