Multiple investigations into animal welfare, as well as into health and safety have taken their toll at Christchurch's Orana Wildlife Park, with eight staff handing in their resignation.
Park Chief Executive Lynn Anderson told Stuff the campaign of unsubstantiated allegations began in March last year.
Letters and photos, containing allegations, have been sent anonymously to the Ministry for Primary Industries (MPI), donors and other external agencies.
"They have related to animal welfare and safety. All anonymous complaints have been investigated by external agencies and cleared," said Anderson.
All complaints received by MPI have been investigated and found to be unsubstantiated, Stuff reported.
Anderson says the photographs part of the complaints could have only been taken by a staff member, with one showing a lion with a fungal skin condition.
"We don't know who is doing this and, for reasons unbeknown to us, if it has been from the inside they have chosen to go to external authorities rather than internal park management."
A full staff meeting was held on May 29, and since then several full time and volunteer staff have been interviewed as part of the Park's own internal investigation.
Eight staff members have quit since the zoo launch the investigation, which Anderson says is a "curious coincidence".
Anderson says in the last 14 months, she has lost sleep, felt helpless and like a "victim".
"It's my job to protect the organisation, the people and the animals and we're just under constant attack by these anonymous complaints.
"It's just not stopping and we're at a bit of a loss, we need this to stop. It's getting to the point that it's so unfairly having potential harm on our organisation's reputation and all the great people in it."
She says the park has continued to operate safely and compliantly, with animal welfare and health and safety key priorities.
"How many times can the same animal welfare complaint be investigated and closed out as not of concern?"
Anderson says the 52-person team will move forward.
"We feel a real sense of making Orana Park the very best it can be for the local community."