Updated with new images from a couple of days later and a disected gall showing the porous, sticky inside of the gumnut-imitating part and the smaller empty woody part that attaches the gall to the leaf.
I was wondering overnight about how to report it as a gall instead of a tree so that it attracts the attention of the insect specialists. If you can change it from tree to insect gall that would be great. It's a very interesting looking structure appearing (and I'm way out of my depth here) to be trying to attract other insects?
A closer inspection of the location on 4th November showed several pairs of these galls on the same and adjacent young E. blakelyi saplings, as well as several incipient galls, ie only the part that attaches the gum-nut-imitating part to the leaf was formed. The same trees had a couple of other types of galls including that posted in my sighting 4526309 and a type matching sighting 4525327.