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Emergency services were called to Shoalhaven Zoo in Nowra on Friday morning after the 35-year-old woman suffered serious head and neck injuries in the attack, a NSW Ambulance spokeswoman told AAP.
NSW Police said the woman was cleaning the enclosure at the time of the attack.
The woman has been airlifted to St George Hospital in a critical condition.
NSW Ambulance duty operations manager Faye Stockmen described the attack as "absolutely harrowing", adding that it was frightening to be the first to walk into the lion's den to treat the woman.
"This is one of the worst jobs I have ever experienced - I have never come across a job like this in my career," Ms Stockmen said in a statement.
"The attack was extremely vicious and paramedics found the woman with severe injuries."
It's not the first animal attack at the zoo.
In 2014, a crocodile dragged a male handler into the water during a school holiday feeding show.
The man suffered a minor hand injury.
Shoalhaven Zoo has been contacted for comment and Safework NSW has been notified of the incident.