The ACT Plant Census only has the following yellow box hybrid: E.mannifera subsp. mannifera × Eucalyptus melliodora. I wonder if that is what you have seen @waltraud
Interesting, thanks Steve. Census doesn't always include species or hybrids that occur for instance the hybrid Acacia baileyana × Acacia decurrens was added only recently (I think based on material that we presented). I think there are more hybrids around especially in the bipinnate Acacia (decurrens - parramattensis ...). The characteristics are often not clear cut and specimens might be difficult to id in the field even for experienced people. It wouldn't surprise me if E melliodora hybridizes with a number of other species.