Staff have confirmed an African lion killed its new mate at the Taronga Western Plains zoo at Dubbo in New South Wales.
Lazarus arrived at the zoo from Auckland last November and was paired with a lioness as part of an international breeding program.
In a statement, the zoo said the pair had had visual and fence contact prior to their introduction to ensure their behaviour towards each other was receptive and positive.
The zoo says interactions between unrelated carnivores are known to be fraught with danger and can lead to death or injury in the wild.
But general manager Matthew Fuller says the two lions appeared to be getting along well until their fight about a week after first meeting.
The lioness suffered serious injuries to her airways and died shortly afterwards.
Mr Fuller says fights are always a risk when working with animals such as lions.
"It is sad news," he said.
"But I guess putting things in perspective, this is something that happens when you're dealing with big carnivores, particularly something as capable and powerful as a lion, these things are well and truly possible."
The fight happened several weeks ago, but the zoo has only now chosen to release the information.
It is currently looking for a new mate for 11-year-old Lazarus.