"To bring tools like bolt cutters is something that we think is quite premeditated," she said.
The intruders used bolt cutters to open a door into the golden lion tamarins' area and then entered the squirrel monkey enclosure.
"They've obviously tried to take a squirrel monkey," said Fifield.
Staff checked the habitat several times before contacting police and issuing a media statement saying the monkey was missing and staff were concerned for her welfare.
However, about 12.20pm a zoo spokesman confirmed the animal had been found, hiding in her enclosure.
"We had one that we thought was missing but she was hiding, obviously very scared," said Fifield.
Although all the squirrel monkeys were now accounted for, some had been hurt during the break-in. The animals are being checked by a vet tomorrow.
The monkeys' injuries indicated they had been grabbed, Fifield said.
"The monkeys would have defended themselves. They're quite feisty little animals and they would not have been very happy about a human they did not know in their habitat."
One monkey had a haematoma on its elbow and others had scratch marks.
The monkeys would have definitely bitten the alleged offenders, Fifield said.
The zoo would not speculate whether the monkeys were being stolen to order and said they did not have CCTV footage of the break-in.
The outside security fence was not breached, but a contractor had left a service gate unlocked.
"However, because [the intruders] had bolt cutters they would have got through that bolted gate anyway."
When asked what prices squirrel monkeys could fetch, Fifield said the zoo did not put monetary values on its animals.
"To us they're priceless."
The break-in had also distressed staff.
Squirrel monkeys were social animals and would be terrified if they were stolen and separated from their mates.
"We want to say to whoever tried to do this, this is really just not acceptable and it's not appropriate to try to take one of these animals," said Fifield.
The zoo said this morning when one of the monkeys was still believed to be missing that it wouldn't have pressed charges or asked questions if the alleged offenders had returned the animal.
Fifield confirmed zoo has launched an internal investigation and would be increasing security patrols.
Wellington City Council acting chief executive Kane Patena said they'd offered support, particularly any resources the zoo might need for the investigation.
"If there's a will, there's a way. You can put all sorts of security measures in place, but if people are intending on committing crimes they will always try to find a way.
"Part of our role and responsibility is to try and get ahead of people who have this kind of will and intent."
Police said they were alerted to the break-in around 9am, and had since examined the scene.
Anyone who has seen something suspicious is asked to ring Wellington Police on (04) 381 2000, or to call Crimestoppers anonymously on 0800 555 111.
Although rare in New Zealand, animal thefts from zoo enclosures have happened all over the world.
In 2015 National Geographic reported golden lion tamarins were stolen by professional animal dealers from Zoo Krefeld in Germany.
The magazine reported that in 2011, 400 animals were stolen from European zoos and in 2015, 25 members of the European Association of Zoos and Acquaria reported thefts.
One French zoo had 79 tortoises stolen.
At the time the director of the Association of Zoological Gardens in Germany, Volker Homes, said zoo thefts were "a really severe problem".
Closer to home, last month Charisma the alpaca was stolen from Dairy Flat, Auckland, leaving his blind mate Bambi adrift.
The alpaca hasn't been found.
In December 2016, the Sun reported a group of teenagers broke into London Zoo at night and filmed themselves exploring the enclosures.
One teenager was bitten by a llama during the escapade, and another jumped into a penguin enclosure.
The escapade was posted on YouTube.
In 2008 Hamilton Zoo recovered a stolen pair of Madagascan day geckos and two bearded dragons.
Thieves cut through chained and padlocked doors to get into the reptile house to steal the lizards.
What is a squirrel monkey?
• According to the Zoological Society of London the primate, also known as Saimiri, is an endangered species that grows to 35m and weigh up to just 1100g.
• They have the largest brain of all primates with a brain-to-body mass ratio of 1:17.
• They live together in large troops of up to 500 and have a polygamous mating system.
• When threatened they make vocal calls, including warning sounds, and have been called "small, nervous primates".
