Sisters Djane and Zahra arrived at Wellington Zoo in April 2002 and were put down a few months shy of their 20th birthday.

Djembe, the third sister of three, was euthanised two years ago.

Staff made the painful decision to euthanise the pair after a sharp decline in Djane's health, which surgery failed to arrest.

"The team had observed dramatic changes to Djane's demeanour and she had become disinterested in her food," animal care manager Jo Thomas said.

Relocating Zahra to another zoo or allowing her to live out her remaining days solo was dismissed "on animal welfare grounds".

Ms Thomas said being euthanised was "the best thing" for her.

"As carers for these animals, their welfare and how they experience life is always at the core of our work at the Zoo and these decisions are never easy," Ms Thomas said.

"Zahra would not be accepted into another pride of lions, and the stress caused by moving an elderly lion, as well as introducing her to other lions would be irresponsible and dangerous.

"Zahra and Djane are litter siblings and share a very strong, sisterly bond.

"Lions are incredibly social and family orientated animals. Zahra's welfare would have been severely compromised if she was left to live as a solitary lion."

Wellington Zoo, New Zealand's first public zoo, has a long history of hosting lions dating back to its opening in 1906.

The first lion was a male named King Dick, inheriting his name from the nickname of then-national leader Richard Seddon.

King Dick, who lived in captivity until 1921, was the zoo's first occupant and the sole exhibit until he was joined by a kiwi, an emu and some monkeys.

Ms Thomas said the zoo was yet to formulate a plan to replace Djane and Zahra but were committed to doing so.

"While Wellington Zoo will be without lions for a while, in time we will look at welcoming a new pride so we can continue our conservation work with this iconic species."

Australian Associated Press