I wouldn't think a "tea" that is non-toxic to human consumption would have any preservation quantities. Eucalyptus or tea-tree probably would but the quantities required to prevent bacteria formation would render the entire mix pretty toxic, which would be counter-productive.
A quick read around the web shows that Red Shiso is mixed with vinegar in order to use the apparent preservative found in the herb. I don't think anyone is using the Red Shiso on its own. Therefore, any extraction of a similar native Australian preservative is still probably going to need mixing with a more traditional preservative, such as alcohol, vinegar, salt, or sugar.
There hasn't been a LOT of research done on the preservative elements of various native plants, unfortunately. It might be possible to find traditional Aboriginal uses of various things to keep foods for longer, but whether they would be suitable in the sort of highly sterile environment your essences presumeably need is up for debate.