One anonymous photo was of a lion with a fungal skin condition and could only have been taken by a zoo staffer. (The healthy lion pictured is a file photo.)

Eight staff have quit the South Island's largest zoo following an internal investigation into a "malicious" 14-month campaign alleging animal welfare and health and safety breaches.

The zoo's chief executive said while all allegations were found to be unsubstantiated, the "constant attack", had taken its toll.

Orana Wildlife Park initiated an internal investigation into the allegations after photos and letters were sent to the Ministry for Primary Industries (MPI), donors and other external agencies.

Eight staff have since resigned.

READ MORE:

* Orangutans settle in at Orana Park

* New scarlet simians make Christchurch zoo home

* New $500k rhino facility at Orana Park

The park's chief executive, Lynn Anderson, told Stuff on Friday the campaign began in March 2017.

"Over the last 14 months there has been ongoing, anonymous letters to a range of external organisations containing allegations about Orana Wildlife Park.

"They have related to animal welfare and safety. All anonymous complaints have been investigated by external agencies and cleared."

An MPI spokeswoman confirmed all complaints received had been followed up and found to be unsubstantiated.

John Kirk-Anderson Lynn Anderson, Orana Park chief executive, said the zoo had been "under constant attack" by anonymous complaints for more than a year.

While the letters were anonymous, Anderson said they contained photographs that could have only been taken by staff.

One photo was of a lion that had 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."

Do you know more? Email sam.sherwood@stuff.co.nz

Anderson said a full staff and volunteer meeting was held on May 29, with those in attendance told an internal investigation was under way.

Several staff were interviewed as part of the investigation and asked if they had seen the photos before.

She said it was a "curious coincidence" that eight staff resigned since the investigation, which remained open, began.

Anderson said she felt like a "victim" over the last 14 months.

"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 said she had lost sleep and felt "helpless" as the allegations continued to come in.

"How many times can the same animal welfare complaint be investigated and closed out as not of concern?"

ALDEN WILLIAMS/STUFF Orana Wildlife Park is the South Island's largest zoo.

Anderson said the wildlife park continued to operate safely and compliantly, with animal welfare and health and safety key priorities.

"We have 52 people employed in the full complex. In terms of zoo culture the majority of our team remain engaged, we have a number of very long-serving staff members and as a team the park is going forward.

"We feel a real sense of making Orana Park the very best it can be for the local community."