In this video from April 2018, Wellington Zoo chief executive Karen Fifield talks about a break-in at the zoo's squirrel monkey enclosure.

Monkeys were found with scratches and bruises, scared, but otherwise OK, after a break-in at Wellington Zoo.

At least two of the zoo's 12 squirrel monkeys were bruised and battered after someone used bolt-cutters to break into their enclosure after the zoo closed on Friday night.

Police began investigating after Wellington Zoo team leader of primates Harmony Neale arrived at about 8.30am and initially thought one of the monkeys had been stolen. However, the monkey was later found to be hiding.

"They're very feisty," Neale said.

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"Their best weapon is their teeth. But they're wriggly and can turn in on themselves. They're very quick."

One of the injured monkeys, which weigh roughly 750 grams, suffered a large haematoma on her elbow while another had scratches likely from fighting back.

ROSA WOODS/STUFF At least two of the 12 squirrel monkeys were injured during the break-in.

The monkeys were timid as visitors walked past the enclosure on Saturday; preferring privacy among the trees and bush.

They will all have full health checks once things calm down over the next few days.

Chief executive Karen Fifield said the would-be thief would have "some monkey bites, for sure" from the "sophisticated and pre-meditated" crime.

The zoo contacted the police immediately and would be investigating internally as well as boosting security patrols.

Its outer perimeter fence was electrified, Fifield said, but a security contractor had left a service gate open.

She pointed out a thief could have used bolt-cutters to get in anyway.

Once inside, they cut a padlock off the door to the two golden lion tamarins' enclosure then went through a zoo keeper door into that of the neighbouring squirrel monkeys.

ROSA WOODS/STUFF Wellington Zoo chief executive Karen Fifield said at least two squirrel monkeys were injured after someone used bolt-cutters to break into their enclosure behind her.

The area was not alarmed, and the internal door was left open. The wind had blown closed the outer door.

"If their intention was to have it as a pet, it was a serious mistake.

"Squirrel monkeys like to be with others, they don't like to be with humans at all.

"If they were scared, which they would've been, they would have screaming and biting.

"It's actually illegal to have these as pets so they'd be difficult to sell."

She said a price could not be put on the monkeys.

"All our animals are special to us. We don't sell them, and we don't put a price on them. With what we are trying to do with squirrel monkeys, they are priceless."

ROSA WOODS/STUFF One of the monkeys was first thought to have been stolen, but it was later found to be hiding.

A police spokesperson said staff were in the "very early stages" of an investigation.

"Police were alerted at 9am [Saturday] morning to a break in at Wellington Zoo in one of the enclosures.

"Staff have been at the zoo this morning carrying out a scene examination."

Police asked anyone with information to ring Wellington Police on 04 381 2000 or Crimestoppers anonymously on 0800 555 111.

WHAT ARE SQUIRREL MONKEYS?

Auckland Zoo describes Bolivian squirrel monkeys - Saimiri boliviensis - as being extremely agile and fast.

Unlike a spider monkey, their tail is not used for climbing, but as a kind of balancing pole. They eat mainly fruit and insects but also invertebrates, flowers, leaves and seeds.

During mating season, males gain a lot of weight. This is known as "fattening". The fatter the animal the more attractive it is to a female.