Hi - I have seen this too - its usually on Olearia paniculata (as I think your image shows) but also on O. angulata, O. albida, O. coriacea and O. furfuracea (that or allied taxa). I assume its a mite or gall midge of some sort but I have never got around to finding out the cause. You could try asking Dr John Early (jearly@aucklandmuseum.com) - send him your image, he may know what causes it, and if not who may know - as gall midges and the parasitic wasps that take them on are his specialty.