Adelaide Zoo's Karta the orangutan and her mate Kluet are mourning the loss of their baby.

Zoos SA senior veterinarian Ian Smith said the exact cause of death was unknown but initial observations indicated the infant was stillborn.

"Similar to humans, complications at birth or within a few days after birth are not unusual for great apes. However, unlike human births, there is less opportunity for medical intervention," Dr Smith said.

"The exact cause of death is unknown at this point as the infant is still with Karta; however, this will be determined in the coming days when we are able to perform a full veterinary investigation."

Adelaide Zoo primate keepers said Karta and Kluet were in good health and staff would continue to assist the pair in their recovery and throughout the mourning period.

"As is the case for many primate species, it is essential Karta and Kluet are given the appropriate time and space to mourn the loss of their infant," the zoo said in a statement.

"Our main concern now is to ensure the pair has the necessary support to help them through this period and to make sure they feel comfortable and secure."

The veterinary team will perform a post-mortem in the coming days to determine the exact cause of death.

Karta, now 31, has lost six offspring since 1995. She was due to give birth in November.

Zoo staff had previously identified that Karta has a problem breastfeeding and that has contributed to her failure to raise her infants.

Karta and Kluet will remain in their night quarters until further notice.

Hundreds of messages of support are being left for the orangutans and Adelaide Zoo staff on their Facebook page.