Photos used in a "malicious campaign" alleging animal welfare concerns at one of New Zealand's largest wildlife parks were of animals under vet treatment and from a work computer, the zoo boss alleges.

Lynn Anderson, chief executive of Orana Wildlife Park in Christchurch, said some of the photos - sent to external agencies including Ministry of Primary Industries and financial donors - were taken of animals to document the healing process.

"Some of the photos were from our computer system that only staff can access," Anderson said.

"Some were taken of animals under vet treatment and they had been taken to check healing and response to treatment."

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Anderson said someone "with something against the park" had been anonomously sending letters and photos for the past 14 months.

Complaints made to MPI were investigated and found to be unsubstantiated.

Eight staff had resigned from the park after an internal investigation into the campaign.

Anderson said photos and letters alleging animal welfare and health and safety breaches had been sent to external agencies and donors.

She could not say with certainty who was behind the campaign.

"Put it this way, we can't say for certain because I don't have any strong evidence at this stage that it's this person or that person," she said.

"But the photos sent to external bodies could have only been taken by staff."

Anderson said animals at the park were well cared for and was at a loss to explain the complaints.

"We don't understand it. Our animals are on display to the public 364 days a year.

"If there was any neglect or issue then the public would see that. You can't hide that sort of thing."

All of the complaints had been made anonymously, Anderson said.

"Anyone with genuine concern for animal welfare would have put their name to it and they wouldn't have gone to our donors.

"If someone is genuinely concerned for animal welfare why would you send anything to our donors who help us provide quality care to the animals."

There were 52 staff at the wildlife park and Anderson said all current employees were engaged and doing their best for the animals.

"I can't get inside the head of the person doing this so don't know how to explain what is going on.

"All we can do is keep doing the job we are doing which is caring for these animals."