Emergency services were called to Shoalhaven Zoo in Nowra on Friday morning, with Jennifer Brown suffering serious head and neck injuries in what was described as an "extremely vicious" attack.

Ms Brown was cleaning the enclosure at the time, with the zoo closed to the public because of COVID-19.

The experienced zookeeper was airlifted to St George Hospital and remains in a critical condition.

NSW Ambulance duty operations manager Faye Stockmen described the attack as "absolutely harrowing", adding 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."

Police Detective Superintendent Greg Moore said the two male lions were secured shortly after the attack and there was no further risk to people.

Asked if the lions would be put down, he said police will look into what happened and it was too early to pre-empt an outcome of that investigation.

"Fortunately, lion attacks are rare in this part of the world," he said on Friday.

"We will be working with authorities to make sure if there's anything we can do to make these types of establishments safer, we will be doing our best to support that."

It's not the first animal attack at Shoalhaven 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.

Australian Associated Press