There are three Sydneys, a CBD area just South of the Harbour Bridge, name Sydney, admin_level 10, a larger area which covers the smaller Sydney boundary. This is the Council of the City of Sydney (short name Sydney), admin_level=6 then the largest area of all extending almost to the Blue Mountains. Its name is also Sydney with admin_level=7.
I did not know until recently when I obtained confusing results from a query. After some trouble, I discovered that there were more than one Sydneys.