The third monkey stolen from a NSW zoo has been dropped off at a vet and is in good health.

The pygmy marmoset, whose newborn baby and daughter were found on Sunday, was on this evening left in a box outside a vet's office at Tahmoor, west of the Symbio Wildlife Park in Helensburgh, from which the animals were taken on Friday night.

"Arrangements have been made to return the monkey to the wildlife park," police said.

One of three rare monkeys stolen from a Sydney wildlife park has been found. (Image: NSW Police)

Earlier, two Sydney brothers involved in the theft have pleaded guilty to transporting and intending to sell them.

Jackson George, 23, and his brother Jesse, 26, were charged with dealing with the proceeds of crime after they were found with one of the monkeys, a four-week-old, while driving in Appin in Sydney's south-west.

Police believe the thieves forced their way into Symbio Wildlife Park late on Friday night and broke into the Pygmy Marmoset enclosure to steal the four-week-old baby, which is too young for its sex to be identified, its 10-month old sister Sofia and father Gomez.

The elder Mr George was fined and given a two-year good behaviour bond at Campbelltown Local Court, while his younger sibling will be sentenced in January.

Symbio zookeepers were gravely concerned about how the theft would impact the complex social structure of the family, but said the baby's reintroduction has been promising.

"Mum cradled the baby straight into her arms and bub immediately began to feed," the Symbio Wildlife Park posted on Facebook.