Should I order references on surname or paper title when using Harvard style?
From Harvard website: 


  The entries in the list should be in alphabetical order by the
  author's last name [sic].


As pointed out by aeismail in the comment, the second criterion is the date:


  Works by the same author should be ordered chronologically, from oldest to most recent


Of course, you might have many articles with the same author and same year, in that case, they say that:


  If your reference list includes multiple works by the same author in
  the same year, identify them in your parenthetical citations and in
  your reference list by a lowercase letter after the year, assigning
  each letter in alphabetical order by the title of the work