The calf of an elephant at Sydney's Taronga Zoo, believed to have died in labour, has been delivered alive.

Porntip gave birth to the calf this morning, zoo spokesman Mark Williams told AAP.

On Monday, staff at the zoo said ultrasounds showed the calf was in an upside down position, making it impossible for the mother to give birth.

Zoo staff previously believed the calf had died in the womb on Sunday night, but now admit they were mistaken.

"Remember you are dealing with a three tonne animal. It's very hard to get absolutes," Mr Williams added.

The male calf was delivered at 3.27am Wednesday.

Zoo director Cameron Kerr said the baby was in a very delicate situation.

"Her dedicated keepers reported the amazing news early this morning that the calf had been born and was showing signs of life," he said in a statement.

"Taronga's veterinary team immediately rushed to the elephant barn to assist Porntip and the calf."

He said the vet team is working to support both mum and baby.

Previously, after six days of intensive labour by Porntip, keepers and veterinarians had found none of the five vital signs that would have indicated the calf was still alive.

The zoo's senior veterinarian and world-leading expert on elephant births, Dr Thomas Hildebrandt, said on Monday it would be a miracle if the calf was born alive.

But following the birth on Wednesday, Mr Kerr said the possible effects of the protracted labour on the calf were yet to be determined, and it may not survive.

"While this is incredible news, the young calf still has a long way to go," he said.

"There are no guarantees of its long-term survival at this early stage but we hope that its birth against the odds will stand it in good stead."

- AAP