The 35-year-old woman suffered severe face and neck injuries on Tuesday at Shoalhaven Zoo, about 150km (90 miles) south of Sydney.
NSW Police said the woman was cleaning the enclosure at the time of the attack.
Four ambulance crews and a rescue helicopter are attending the scene and paramedics are preparing to airlift the woman to Sydney Hospital.
She suffered "extreme accidents" to her head and neck and is in a vital however secure situation after being saved by two quick-thinking colleagues.
Inspector Faye Stockman said zoo staff accompanied paramedics into the lions' den.
When 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.
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Two ambulance crews and a specialist medical team had to "walk into a lion's den" to treat the injured woman at Shoalhaven Zoo in North Nowra.
Speaking to reporters, Mr Moore mentioned police had been starting to research how the incident unfolded, South Coast Register reported. He is thought to have been working at the zoo as an animal handler for around a decade when the incident occurred.
A spokeswoman for the zoo instructed information.com.au on Friday afternoon no remark was being made concerning the state of affairs at the moment.
"Come nose to nose with our Roarsome Foursome and get to feed the 'King of the Savannah",' the web site reads.
The zoo is closed to the public because of coronavirus pandemic.
In July 2014, keeper Trent Burton was dragged into the water by a 3.7m crocodile during a lunchtime feeding demonstration at the zoo.