Mark Twain tells the story of the wreck of the Duncan Dunbar in chapter
9 of Following the Equator.  He is in error.  The name of the ship
wrecked approaching Sydney harbor was the Dunbar.  It was owned by
Duncan Dunbar.  All other details seem to be true regarding this
unfortunate event of 20 August 1857.  There was a wreck of a ship named
Duncan Dunbar, however.  Owned by the same company and also headed for
Sydney.  This one wrecked off the coast of Brazil 7 October 1865.  (All
this according to Wikipedia and the New South Wales State Records).