This should be in the Pests/Diseases forum !!
As a general rule this type of skin disease begins due to high humidity and low air circulation. A few things will help.
Water the plant less.
Move to a location with better air flow, especially as I see your in Brisbane the humidity will be high and air flow will be the key to stopping this reoccuring.
If the humidity will remain high you will need to apply a systemic fungicide.
Thats should sort it