These sub-conical galls with apical holes are female galls of a gall inducing scale insect. The female galls showed enlarged leaf glands. Some leaves also showed small cylindrical galls with apical openings- these were the males of the same species of scale insect (pic 3). Pic 2 shows galls of both sexes on the one leaf.
The underside of the leaf showed slight discolouration and minimal waxy bloom (pic 4).
Spotted on juvenile leaves of a eucalyptus tree (species unknown) in a nature reserve.