Looks like a powdery mildew like fungus on Wahlenbergia. Quite severe in one area. Have not seen it before but it wiped out several square meters. A mildew has been common plantain in HGW this year - fungus not identified. I sincerely it does not become an important problem for the Wahlenbergia!
There is a recent paper by Levente Kiss (and many others) in Frontiers in Microbiology 16 July 2020 and can be found at https://www.frontiersin.org/articles/10.3389/fmicb.2020.01571/full and titled - Australia: A Continent Without Native Powdery Mildews? The First Comprehensive Catalog Indicates Recent Introductions and Multiple Host Range Expansion Events, and Leads to the Re-discovery of Salmonomyces as a New Lineage of the Erysiphales. In this paper there is no record of powdery mildews from Wahlenbergia in Australia, they have been recorded on the Campanulaceae overseas. I have not worked on this group bests I can do is provide you with one of the papers authors email details.