Prime Minister Jacinda Ardern takes a briefcase packed with documents home each night, but this morning she was surprised with an edible version.

Ardern received a birthday cake decorated to look like a briefcase after celebrating her 40th birthday yesterday.

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.

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

Jacinda Ardern was surprised with an edible briefcase for her 40th birthday. Photo / Supplied
Jacinda Ardern was surprised with an edible briefcase for her 40th birthday. Photo / Supplied

"Thanks team, very grateful to work with such an amazing group of people."

Ardern has tried her hand at a few creative cakes of her own in the past, creating both a piano cake and a bunny cake for her daughter Neve.

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