A number of squirrel monkeys were hurt overnight at Wellington Zoo after a break-in.
The zoo initially thought one of the monkeys had been stolen, but later found it hiding in its habitat.
Two other monkeys were found in another species' habitat - a situation the zoo said "could easily have ended in disaster". The "forethought and sophistication" that went into the break-in was upsetting, it said.
"Some of the monkeys have been injured," it said in a statement. "We will continue with the police investigation and an internal investigation into how this has occurred."
Wellington Zoo Trust chief executive Karen Fifield said if the would-be thieves had succeeded in stealing a monkey it would have been incredibly upsetting.
"We know that they're social animals. They need to be with their mates. It's not good having a monkey on its own.
"If had been gone it would have been so terrified. That was quite distressing to all of us."
The monkeys were all too afraid to be captured on Saturday, but in zoo staff will perform health checks on the animals in the coming days.
One had a visible haematoma, while others appeared to have scratches.
According to Ms Fiefield the monkeys were "feisty little creatures" who would have fought back, leaving the 'monkey-nappers' with visible scratches and bites.
"The monkeys obviously would have defended themselves," she said.
"They would not have been very happy about a human that they did not know in their habitat so they would have definitely fought against being taken, hence the injuries."
Anyone with information on the break-in is urged to contact the zoo on (04) 381 6755 or contact Wellington police.