"It's not a nothing birthday, every birthday should be celebrated," Clarkson told her.
Over the weekend, a Twitter use spotted the Prime Minister out in public and her fiancé Clarke Gayford revealed why she was spotted at a pedestrian crossing.
"I love that NZ is small enough that I see the PM at least once a month," the person posted.
Gayford replied, revealing she was seen out on her own because of a birthday surprise from him and their daughter Neve.
He wrote: "Haha, she was out in the wild here because Neve and I had banished her while we snuck off to get her birthday present."
For her 40th birthday last year, the Prime Minister received an epic "briefcake".
The cake was complete with edible Cabinet papers and a happy birthday tag.
"It seems entirely appropriate that this was the amazing cake the office blew me away with this morning," she wrote on her Instagram at the time.
"Every night they send me off with one of these briefcases, an edible one made for a very nice change!
"She was out in the wild here because Neve and I had banished her while we snuck off to get her birthday present."
Ardern and Gayford are planning a summer wedding, and she revealed earlier this July the couple didn't have a honeymoon planned.
Ardern and Gayford got engaged Easter 2019 at Mahia during a break at Gayford's family bach.