A lion at the Zion Wildlife Kingdom had to be put down after vets could not discover what was wrong with the animal.
Under the operator's licence issued by the Ministry of Primary Industries, park operators are not required to inform the ministry if they are planning to euthanise an animal.
Martin Weekes, a director of Bolton Equities that bought the business in early 2014, said people needed to understand animals in the enclosures were getting old and would have issues in future. He said a tiger which had a lump in an eye was recently operated on and was expected to make a full recovery, while the lion was put down in the past few weeks.
"When an animal becomes sick, as was the case with this lion, we run blood tests, urine tests and the animal just wouldn't eat and becomes more sick.
"If you can't find out what happened to it, do you let it suffer or you do the most humane thing and put it down? Some were declawed by Craig (Busch, former operator) and they'll have difficulty walking properly and can't climb as it puts pressure on their feet," he said.