Staff members are devastated by the sudden death.
"I have worked with Harold since he arrived here (as a 1-year-old) from Chicago in 1994," said animal collection manager Ian Adams.
"He was an absolutely iconic animal and his death is a major loss to the park. It has come as a shock to us all as he was not an old animal.
"He had a relatively easy-going manner for a bull and will be sadly missed. He was a true gentle giant of the park... so many visitors will have had the joy of hand feeding him."
Final results of the animal autopsy won't be returned until lab results are returned next week.
But the cause of death had been confirmed as food in his lungs, the zoo said.
The average lifespan of a wild giraffe is 20-25 years, but animal in captivity can live as long as 28 years.
"He definitely went earlier than he should've. So we do await the results on his fourth stomach chamber with interest," Ms Anderson said.
Harold had eaten normally yesterday, but as he retired into his house for the night he appeared disinterested in food.
"That was unusual for him, since he was an animal that loved his food," she said.
Harold sired nine offspring and will also be remembered for his contribution to the zoo-based breeding programme for his species.
"We have three giraffes left and one of the females is pregnant with Harold's calf and she will be giving birth around November/December. So, we'll have a Harold junior perhaps."
Harold arrived in Christchurch from his birthplace of Chicago in 1994.
He was named after mascot of the Life Education Trust, a charity that aims to provide children with the knowledge to make informed choices about their health and respect others.
It reaches 225,000 primary and intermediate children every year with a giraffe called Harold featuring in its publicity material and visits schools to listen to children and "be their friend".
Jamie Simpson, chairperson of Life Education Trust Canterbury, said it was a "sad day".
