The 21-year-old national criterium Champion, who for the past two seasons has raced for Team Tibco in the United States, took charge of the 123km race at the halfway stage and never looked back.
In the last of the eight laps she had to withstand a bold challenge from former Dane, Linda Villumsen who has switched her allegiance to New Zealand. Villumsen chopped a deficit of more than six minutes in half as she turned in a brilliant performance on the gutbusting Dyers Pass Rd hill climb. However, she left her run too late and finished 3min 6sec behind the Te Awamutu woman.
"I'm ecstatic," Buchanan said. "I didn't think about the finish line until I turned the corner for the last time. I didn't think for one second I'd be able to win the title but I wanted to go at my own pace uphill and tried to keep the tempo going.
"I left everything out on the road today … this is the biggest and most important win of my career."
Villumsen found the hill climb on the course based on that used for the 1974 Commonwealth Games road race "a killer".
"I didn't feel so good out there today; it was a struggle. As well as the hill I was still feeling the effects of the time trial yesterday."
She said with no-one from the peloton pushing the pace it became very difficult to make inroads into Buchanan's lead.
Third place-getter Kaytee Boyd, of Auckland, was thrilled with her showing. She said she and Villumsen talked tactics during the race and decided to share the job of trying to catch Buchanan.
"I went first, got caught and then Linda attacked. When she'd had enough, I took another turn and it was good teamwork.
"The hill climbs weren't as hard as I thought. Most of the girls I know were scared of them, but it's all about how hard you can suffer."
Boyd finished 1min 05sec behind her new Auckland team-mate.
The first significant breakaway occurred in the second lap when national points series champion Serena Sheridan stepped up the pace. The Hawke's Bay rider who represents East Coast North Island opened up a gap of 2min 06sec on the peloton by the start of the third lap.
By the end of this lap she was being pushed by Aucklander Marina Duvnjak and Buchanan. At the halfway stage the two challengers caught Sheridan who hit the wall on her next hill climb and faded.
Buchanan then hit her straps with Duvnjak more than 2mins in arrears and another 4min 35secs to the bunch. By the bell lap Buchanan enjoyed a 6min 40sec advantage over Villumsen and Wellington's Rachel Mercer but she had no chance to cruise as Villumsen wound up for a last-ditch assault.
Thirty-six riders contested the event. Waikato/BoP's Sonia Waddell finished 11th, national time-trial champion and defending road race champion Melissa Holt was 16th and Sheridan finished 21st. Canterbury's Joanne Kiesanowski was among the late withdrawals. She suffered mild concussion after a fall mid-week.
The men's race over 186kms on the same course starts at 10.00am tomorrow.
Provisional results, RaboPlus National Women's Road Cycling Championships, 123kms:-
Rushlee Buchanan (Waikato/BoP) 3:46.15, 1; Linda Villumsen (Auckland) at 3min 06sec, 2; Kaytee Boyd (Auckland) at 4min 11 sec, 3; Rachel Mercer (Wellington) at 4min 28sec, 4; Karen Fulton (Tasman Wheelers) at 5min 02sec 5.