The anonymous author claimed to be a staff member at the zoo.
In the letter, seen by 1News, several allegations about the zoo were made, along with a call for an independent review of its board and management.
The park's chief executive Lynn Anderson has denied the allegations, telling 1News she strongly refutes the letter "in its entirety".
In a copy of the letter obtained by 1News, the author requested protection under the Protected Disclosures (Protection of Whistleblowers) Act 2022 and said they felt "fearful of repercussions".
1News reported the letter writer claimed they felt intimidated and staff felt put down and humiliated by making suggestions for change.
The letter also alleged there was a toxic culture at the zoo and health and safety problems with its equipment.
It also raised concerns about animal welfare, claiming the park's lions are tired, overworked and should be retired, 1News reported.
In response to 1News, the zoo's chief executive said the letter contained "multiple unsubstantiated allegations" and claimed many of the statements were factually incorrect.
"It is disappointing that a single anonymous individual has taken steps that are clearly intended to damage the park's reputation and defame our people," Anderson said in a statement to 1News.
The park also said it complied with all relevant legislation and was subject to external audits.
Orana Park has previously indicated it is struggling financially.
Last month it told 1News it wanted $1.5 million a year from the council to help take care of the 1000 animals it has.
Christchurch City Council interim chief executive Mary Richardson told 1News the letter this week was the first time she had heard of other potential concerns.
She said her team would look into it.
