There was one up in Woodbridge in 2009 if I recall.
They must be escapees as I think they are native to W Australia?
I saw a family of 5 in the Veerse Meer in about 2004, swimming in tight formation line ahead. The parents at head and tail of the column were incredibly protective and aggressive.
Black swans are the national bird of Western Australia and can be seen on that state flag. They are common in both southeast and western Ozzie. Largely monogamous many of their pairings are male/male hence they have trouble reproducing. Now there's a surprise.
We used to have one in Lincoln but have not seen it for a few years now.
One appeared on the Trent shortly after ours disappeared so perhaps he moved on. Was almost always seen with a pair of Mute Swans and appeared to help them raise their young.