Clarke Gayford took to Instagram to wish his partner, Prime Minister Jacinda Ardern, a happy 40th birthday.

"I don't really do these posts, but I think a bit of symmetry like the number 40 is worth posting on," he said.

"Happy Birthday today to my favourite achiever in the whole wide world, someone, who despite everything on her plate, still finds time to squeeze lots of fun, love and happiness into our wee family unit. The. Best. xx"

On the Labour Party's Instagram page, fans of Ardern also shared their birthday greetings with one saying: "Thank you for being born."

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Finance Minister Grant Robertson and Justice Minister Andrew Little were among the first people to publicly wish Ardern happy birthday on her special day.

"Happy birthday to Labour Leader and Prime Minister Jacinda Ardern," Little posted on Facebook.

"She was surprised with a cake and birthday singalong at Newton Markets yesterday."

Also attached was a video of the singalong and a surprised Ardern thanking everyone and cutting the cake.

Robertson posted on Instagram along with a few snaps of the pair together.

"There have been good times, crazy times and sad times but it's always been my honour to stand shoulder to shoulder with my friend @jacindaardern," he wrote.