No further details about the patient could be released, she said.
The ambulance service arrived at the zoo at 11.11am after being called just after 11am.
Fire Service spokesman Scott Osmond said St John called firefighters to the scene at 11.07am.
"All I'm aware of is we got a job sent to us from ambulance to assist in any way we can.
"There has been a reported attack of a tiger on a zookeeper at Hamilton Zoo."
St John spokeswoman Teneale Lawrence said ambulances were on the way to the scene.
There had not been an update on the patient's condition, she said.
St John was called about the incident at 11.02am. A veterinarian then arrived at the scene.
The zoo itself doesn't appear overly busy, with less than a dozen vehicles in the car park.
Hamilton Zoo visitor experience manager Dave Smart fronted media outside the front of the zoo's reception earlier.
However he declined to comment on the status of the zoo keeper, how serious his injuries are, other than to say there were currently no safety risks inside the premises.
St John tweeted that a patient was in a critical condition after an "animal attack" in Rotokauri in the Waikato.
Two ambulance vehicles had attended at 11.02am to treat one patient. "No transport was required."
Adam Rich, who is visiting the city from Hamilton, said he saw a keeper open the gates so the Tigers could go outside not long before the attack happened.
"I saw the Tigers about 45 minutes to an hour before the evacuation of the premises and yeah they looked fine, there was a keeper in there who opened up the gate so the Tigers could leave the inside area to go to the outside area but that's all I saw." Zoo staff then approached him and asked him to leave.
"They seemed a bit panicky. I thought an animal had escaped but they guaranteed that an animal did not escape."
He then heard an announcement from zoo staff asking all visitors to leave.
All visitors were then offered refunds.
Hamilton Zoo is located on a 25ha site in the northwest suburb of Rotokauri.
It was founded in 1969 as a game farm and is owned by Hamilton City Council.
According to Wikipedia, in January 2000, construction started on a sumatran tiger exhibit. It was completed in January 2001 and cost $430,000. It features a large pool, climbing structures and bamboo glades.
In March 2001, 14-month-old Sumatran tiger siblings, male Jaka, and females Mencari and Molek, arrived from Wellington Zoo.
Although not originally intended to breed, the death of their sister, Nisha in May 2006 caused the species coordinator to recommend one of the Hamilton Zoo females to take her place.
As the more confident female, Molek was chosen and she was sent to Auckland Zoo that September.
In July 2012, Hamilton Zoo received a four-year-old tigress, Sali, from Dreamworld on the Gold Coast. She is the zoo's potential breeding female after Mencari was speyed in 2010.
In November 2013, Hamilton Zoo sent their male, Jaka, to Auckland Zoo in exchange for their male, Oz.
People have posted messages of support on Facebook, in the aftermath of the incident.
Selena McMinn wrote: "My thoughts are with the Keeper and Tiger involved in this mornings incident. I hope everyone is okay. Stay Strong."
Karollina Marfell agreed: "and to those that may have witnessed this including emergency support service people there at the moment", she wrote.
Twitter user Liz Clark said it was tragic news: "Heartfelt condolences to her," she wrote.