A wildlife park is desperately searching for three pygmy marmoset monkeys that were forcibly taken from their enclosure at a wildlife park south of Sydney.

Staff realised the adult male, young female and four-week old baby were missing from their enclosure at the Symbio Wildlife Park in Helensburgh this morning.

Police and staff are particularly concerned as the baby needs to be with its mother to feed.

"The mother that is left behind here is very, very stressed and the twin baby that they didn't take is with the mother and there's the potential that she won't feed," park owner John Radnidge said.

"Whilst we're concerned about all of them, it's the baby that is the critical issue.

"It hasn't been weaned off mum and she's reliant upon her mother's milk for survival and without that the potential outcome could be her demise.

"They require specialised care, specialised diets, specialised veterinary care and that can only be provided by experts in the field such as people like us who run zoos."

The pygmy marmoset is native to South America and one of the smallest primates in the world, weighing just over 100 grams.

They are urging anyone with information to come forward.

In 2010, eight endangered monkeys were stolen from Symbio, including a breeding pair of cotton top tamarins, their two six-month old babies, and four pygmy marmosets.

In that instance the thieves were believed to have used bolt cutters to enter the enclosure, then captured the monkeys with nets.

All but one were recovered and two men were charged over the theft.