Photo: © 2010 Taronga Zoo

The calf was thought to have died in the womb earlier this week, when zoologists could find none of the five vital signs that would have indicated the calf was still alive.

Dr Thomas Hildebrandt said at the time "should the calf be born alive, it would be a miracle".

So zoo staff got a major and pleasant surprise overnight when they welcomed their latest resident.

But the calf is weak and zoo staff fear for his long-term survival.

"While this is incredible news, the young calf still has a long way to go," says zoo director Cameron Kerr.

"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."