The four-week-old baby was one of three rare pygmy marmoset monkeys taken from the Helensburgh zoo, after entry to their enclosure was forced.

Police were called to the park on Lawrence Hargrave Drive at 8am on Saturday after staff discovered the monkeys were missing.

An adult male and a young female were also taken. But the baby "is at a particularly fragile age and needs to be reunited with its mother in order to be fed", police said.

The stolen male is believed to be Gomez, who was introduced to female pygmy marmoset Iti last year when she arrived at Symbio from Auckland Zoo, as part of an endangered species captive breeding program.

Pygmy marmoset monkeys, native to South America, are the world's smallest monkeys. The size of a human thumb at birth, the babies need constant attention and must feed every couple of hours. The adults require a specialised climate and diet.

According to the San Diego Zoo their "largest threat is the pet trade, due to their tiny size, cuddly appearance, and appealing face. We cannot express this enough: monkeys do not make good pets."

In Australia, exotic animals such as monkeys can only be held by people with an appropriate licence, usually only for exhibition or conservation purposes, and cannot be sold commercially or kept as pets by private owners.

It is not the first time Symbio Wildlife Park has been hit by thieves. 

In May 2010, four pygmy marmoset monkeys and four cotton top tamarins were taken from their enclosures. Three of the tamarins were recovered soon after, dumped in a park in Auburn. Several days later, the four pygmy marmosets were left in the care of an Auburn vet.

Two Sydney boys, aged 17 and 18, were later arrested and charged over the theft. But a male tamarin called Rico remained missing.

Staff at Symbio thought they had cause for celebration when a tamarin was brought to them in February 2011 by a Victorian man, who said a family friend had bought it for him over the internet.

But the tamarin was not Rico. It turned out to be a female named Conchetta, stolen from Brisbane's Alma Park Zoo the previous October.

​Police are now appealing for public help to find the three monkeys stolen from Symbio on Saturday and urge anyone with information to call Crime Stoppers on 1800 333 000.