Werribee Open Range Zoo in Melbourne's west said on Friday it was mourning the death of 11-year-old Thembi, a "gentle giant" who died Wednesday night.

Thembi was found by keepers the following morning between the giraffes' night and day dens, the zoo said in a statement.

Results from a necropsy suggest Thembi was startled, which disrupted his normal blood flow, likely causing sudden heart failure.

"It is believed he died quickly without prolonged suffering," the zoo said.

Zoo director Glen Holland said staff were heartbroken at the "unforeseen and unprecedented incident".

"Thembi was our oldest giraffe and a visitor favourite," Mr Holland said.

The zoo says it will conduct a full review into the death.

Australian Associated Press