The course featured two climbs - one 400 metres long at an average gradient of 7.9 per cent, and the other 700 metres at 4.6 per cent, and Buchanan finished strongly to come close to Williams' time of 28 minutes and 50 seconds.
But despite Buchanan's late push, Williams had timed her effort well to hang on, and said it was a relief to defend her title.
"There's always a bit of pressure coming in as defending champ. It was super hard, I went out a bit too hard on the way out, with the tail wind I got a bit excited, so I was struggling down the back in the headwind, but I pushed through."
The Mitchelton-Scott rider paid tribute to Buchanan for giving her another close contest, but is excited to wear the New Zealand national champion's jersey overseas once again.
"It's always a close battle, but a good battle between me and Rush.
"It's really cool to represent New Zealand over in Europe."
Williams is set to pull on the white and black jersey in time trials at the pinnacle of women's cycling, the Giro Rosa, which she missed in 2018 due to injury, and is also aiming for a strong performance at the World Championships, having finished 11th last year.
Jenna Merrick claimed the Under-23 title, beating defending champion Georgia Christie by four seconds.
