Ryu, this garden shrub may also be one of the Hybrids of Hypericum pabulum, such as H. X moserianum. It belongs in the St Johns Wort genus and has a few weedy close relatives. Did you observe any self seeding? If so please take a herbarium specimen to the CSIRO herbarium, as it may be a sleeper weed of the future.
I had a closer look yesterday but could not find any self seeding. However, there were some flowers so I have submitted that as a sighting (http://canberranaturemap.org/Community/Sightings/Details/2459553), and maybe this will help with a more exact ID.
If further down the track I do find any self-seeding, what are the best procedures for collecting, and getting a specimen to CSIRO?