Adelaide Zoo's 34-year-old Sumatran orangutan, Karta, has died just hours after delivering an infant that was believed to be stillborn.

She went into labour about 6:00pm on Thursday but died later in the night following the tragic delivery.

It is not certain if the infant died during birth or was already dead.

Karta had lost six other infants since 1995, most recently giving birth to a stillborn infant in 2015.

Zoos SA said the exact cause of Karta's death would not be known until a full post-mortem examination was undertaken on Saturday.

Senior veterinarian Doctor Ian Smith said complications after birth were not unusual for great apes.

"However, unlike human births, there is less opportunity for medical intervention," he said.

"An ultrasound was performed on Wednesday which showed a strong heartbeat and no signs that the baby was in distress.

"We're all devastated by this heartbreaking loss."

Karta fell pregnant in 2016 despite being on contraceptives at the time.

Karta's partner and father of infant present at death

Zoos SA chief executive Elaine Bensted said keepers were monitoring her partner, Kluet, who was with her when she died.

"It is similar to a human [and] we expect he will go through a grieving process," she said.

"He is obviously aware of the loss of Karta because he was with her."

She said Karta's death had been a bitter blow for staff who had nursed her through her latest pregnancy, which included night shifts and regular ultrasounds.

"The staff had invested enormous efforts. They're obviously shattered," Ms Bensted said.

The late Karta (left) and her partner Kluet had seven failed attempts at parenthood. ( ABC News: Tony Hill )

Ms Bensted said Karta was a big part of the zoo and her death was like losing a family member.

"She appeared to give birth fairly quickly, then a couple of hours later she passed away," Ms Bensted said.

Adelaide Zoo has previously said there are less than 7,000 Sumatran and Bornean orangutans estimated left in the wild, with fears they could become extinct in their native habitat within the next 10 years.

It said the population had been declining by as many as 1,000 a year.

"This wasn't a planned pregnancy and we had thought long and hard about whether to continue the pregnancy," Ms Bensted said.

"But because orangutans are so critically endangered we made the decision to continue with the pregnancy and give her every support."