I have 12 sick Eumundi Quongdong Trees. They have this weird growth on their leaves. They are still producing new leaves but generally are not healthy. We have had excessive amounts of rain, I am not sure if this has contributed to them not being healthy.
That looks like some sort of insect or larvae that has burrowed into the leaf and is feeding on the sap as it grows. I suggest you take a few leaves to your local nursery where they can quiz you on your local conditions and suggest the best treatment.
Here's some extra reading http://www.ausbushfoods.com/bushfoodsonline/reports/Fact%20Sheets/pirsaquandong.pdf