The three-time road race champ, who is equal on most wins with fellow Waikato rider Melissa Holt, played down pre-race any talk of winning because the field of 30 would have had fresh legs after only 10 of them did the 23.2km time trial on Friday.
But Buchanan held off fellow NZ reps Georgia Williams and Jo Kiesanowski in a sprint finish of the 117km race in 3:35.44, after they rode the final two laps together on Saturday.
She saluted national track coach Brendon Cameron, of Cambridge, for letting them compete at the nationals.
"What Brendon is giving us at training is paying off and he knows what's best for us," she said, emphasising the 3.5-hour on the road now has to be condensed into a 4.5-minute track gut buster.
As the only child in the family, she was delighted to have her parents, Viv and Don, and husband Adrian Hegyvary (pronounced "hisvie") cheer her on here.