Smith earned the honour after making it into the breakaway long enough to outsprint fellow escapees, veteran Sylvain Chavanel (Direct-Energie) and Michael Gogl (Trek Segafredo) atop the Côte de Pouzauges.
His win on the category four bump put him on equal footing with overnight KOM leader Kevin Lédanois (Team Fortuneo-Samsic) but took the jersey on count back as he sat over three minutes ahead of the Frenchman on GC.
Smith seemed a tad shocked by the honour.
"I wasn't sure on that fact (ed-first leader's jersey for New Zealand) I was telling the camerman it was 90 per cent true, now you've confirmed it," he told SBS' Dave McKenzie.
"It's really cool. A real special moment. Proud to be a Kiwi and represent Wanty at the same time."
Asked what he'd want to say to people back home in New Zealand, he said "they'll be sleeping at the moment but when they wake up hope they're watching and proud of me."
"Tomorrow's the team time trial so I should obviously keep it to the next road stage."
It was a good day for the pro-conti Belgian outfit with Timothy Dupont finishing sixth in the final sprint.
"I can not be disappointed with this sixth place," Dupont said. "Especially when I see who finishes in front of me. In general, I feel stronger and stronger during a race in stages. But I've never raced a three-week race, so you have to see what it's going to be like. I have a lot of confidence. "