A black-handed spider monkey has been born at Wellington Zoo for the first time in 29 years.

Conservationists overseas are among those celebrating the new, rare arrival, a boost for the critically endangered species.

Zoo keeper Cameron Hawken said he's "really, really excited, really happy."

"Typically with a lot of animals, they'll actually give birth at night, it's a lot safer for them," he said.

"But she gave birth in the middle of the day, we got a call at 3 o'clock.

"A very strange call that there was a birth down at the Monkey Island, so we rushed down and yeah, baby, very unexpected."

It comes after multiple breeding attempts at the zoo were unsuccessful.

"Because they're an endangered species, it's pretty significant," Hawken said.

"It's just showing that these guys are really well settled in there group here."

And Sarah Otterstrom of Paso Pacífico, a US conservation organisation, joined those celebrating the birth.

"This is a huge achievement for conservation, and for building awareness about the importance of this species in the wild."