A pygmy marmoset, the world's smallest species of monkey, has given birth to twin monkeys at a New South Wales zoo, weighing in at just 15 grams.

The tiny monkeys were born at Symbio Wildlife Park in Helensburgh on Monday.

Their weight of 15g may seem tiny — roughly the same as a CD — but compared to the already miniature size of their mother, these babies are heavy.

"If you put into perspective it is actually relative to a human giving birth to a 10-year-old child," the Zoo said alongside a YouTube clip of the new family.

The average weight of an adult pygmy marmoset is 100g.

Parents Gomez and Iti coupled-up in mid 2015, after the zoo set out on a match-making mission to put a halt to Gomez's bachelor ways.

Gomez's "soul mate" came to Australia from Auckland Zoo in New Zealand.

The zoo said the first-time parents had been "inseparable since the outset".

Pygmy marmoset's are the world's smallest monkey and one of the smallest primates.