Prime Minister Jacinda Ardern turned 40 yesterday and says she spent the day "having some quiet time" and reading Cabinet papers.

Asked about her birthday during this afternoon's presser on funding for educational institutions, Ardern described how she spent her special day.

"Like everyone else actually, a bit of cake and a bit of quiet time," she said, before adding: "that's actually not true, not like everyone else. I don't think everyone else probably reads Cabinet papers on their birthday, but I did."

Known among those who work with her for carrying a briefcase full of documents home each night, Ardern was this morning surprised with an edible version.

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The PM shared a snap of the "briefcake" on her Instagram page.

"It seems entirely appropriate that this was the amazing cake the office blew me away with this morning," she wrote.

"Every night they send me off with one of these briefcases, an edible one made for a very nice change!

In the presser, she said the cake was "fantastic".

"A little heavy on the fondant, as you can imagine," she added.

Her partner Clarke Gayford also took to Instagram yesterday to share a birthday tribute to his "favourite achiever".