It was the second New Zealand title for the Te Awamutu rider, who also won in 2010.
After relinquishing the lead to Aucklander Linda Villumsen with 800m remaining, Buchanan found another gear, using her track speed to good effect to sprint her way to victory in 3hr 32min 25sec.
Villumsen, who was second in the time trial on Friday, had to settle for the runner-up spot again. Otago's Reta Trotman was third.
''I just had to give it everything today. Then Linda passed me and you just don't even think about it. You just go somewhere you can't go any other time,'' Buchanan said.
''It's a pretty emotional win.
''I won it in 2010 and I haven't really been back at my best since then. I'm on a new team this year, and with the Commonwealth Games this year as well it means a lot.
''The track riders have performed this weekend and it's a good way of putting our hands up and saying yes, we're ready.''
Villumsen was second in the same time as Buchanan, with Trotman 7sec back in third.
''It is quite hard to be competitive this early in the season but while I was surprised with my form in Australia last week, I didn't have the legs yesterday [Friday],'' Villumsen said.
''I want this national jersey so I am disappointed today. I thought Rushlee was gone when I went past but she really did well to get back on my wheel ... I just have to come back again next year to win it.''
