Dubbo hippo Mana has died after a sudden acute heart failure.
Taronga Western Plains Zoo made the announcement on Facebook on Wednesday night.
The 26-year-old hippo experienced a medical episode on his paddock at 2.30pm.
"The zoo's veterinary team was immediately called to the scene however the episode was so significant that it resulted in loss of life," the post said.
The zoo said the loss of Mana was "a huge shock to the entire zoo team, especially the dedicated keepers who cared for him for so many years. It is a very difficult and distressing time for all involved".
"Mana was close to our hearts having been born here at the zoo in March 1996. Keepers describe him as a gentle giant," the post said.
"He was the zoo's largest bull, and as our breeding bull he made a significant contribution to the regional breeding program. His legacy lives on through the three calves he sired; females Kibibi and Kendi and male Kani."
The zoo described Mana as a hippo with a "great disposition with an extremely patient and gentle nature and instilled awe in those who met him".
The post said keepers and guests alike enjoyed interacting with him and he would be greatly missed.
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