The rare birth of twin giraffes at Auckland Zoo came with a bittersweet blow after one of the calves had to be put down.

The twins were born to 7-year-old mother, Kiraka, on the morning of New Year's Eve.

The zoo said it was extremely rare for giraffes to have twins, with a little over 30 documented cases worldwide.

But it was also bittersweet announcement, as the twins were born premature with relatively light bodyweights.

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The female calf managed to stand and start suckling quite early, but the male was very weak.

It was unable to suckle or function independently and did not form a bond with Kiraka.

"Despite the great efforts of all involved, the male's post-birth complications became untreatable and we took the very difficult decision on welfare grounds to euthanise the young male calf yesterday morning," Auckland Zoo mammal curator Warren Spencer said.

"Things are looking much more positive with the female calf; she has developed a strong bond with her mother and we are optimistic for the days ahead."

Auckland Zoo's Pridelands and vet teams will be continuing to monitor the female calf closely.

The calves were born at 2.30am on December 31.

Their father was 19-year-old Zabulu. It was mother Kiraka's third pregnancy.

Giraffes have a gestation period of 15 months. At birth they drop some 2m out of the womb with the mother standing up.

Calves are usually able to stand up and nurse within 30 minutes after birth, the male was unable to.