Christchurch's Orana Wildlife Park's management believe a current or former staff member is behind false allegations being spread about the zoo.
Dating back to March last year, anonymous complaints about animal welfare have been made to the Ministry of Primary Industries, donors and other organisations, but all were found to be unsubstantiated.
The allegations included a photo of a lion with a fungal skin condition, which Orana's management said was under veterinary care.
Orana Park chief executive Lynn Anderson said the photographs could have only been taken by someone in close proximity to the animals.
Ms Anderson said eight staff resigned following an internal investigation, but she could not be sure any were directly related to the claims.
She said she just wanted the accusations to stop, after receiving another call from MPI with fresh complaints.
"We took the inspectors to show them all areas and animals of concern that had been raised in the anonymous complaint, and that's what I want to reinforce, is that Orana is transparent and open," she said.
She said the complainant was now sending letters to the media.
While the claims have been found to be untrue by MPI, the accusations could damage the park's reputation, she said.
"When complaints like this are sent to park donors it's a situation where of course it's going to potentially damage the relationship with the park, who relies on public funding and so relationships need to be rebuilt and talked through," she said.
She said whoever was making the allegations should contact zoo management with their concerns.