When is it safe to walk into a lion enclosure? When a zoo digs a giant tunnel beneath their enclosure and builds a people cage on top.

Welcome to the Predator Experience, a $1.4-million plan by South Australia's Monarto Zoo based on the success of shark cage diving off Port Lincoln.

An artist's impression of the lion cage at Monarto Zoo. ( Zoos SA )

In a role reversal of a regular zoo exhibit, it would allow up to 30 visitors into the cage for a close up experience with the lions while they roam free outside and are fed.

Zoos SA chief executive Elaine Bensted said it was "going to give visitors a chance to quite literally go into the lions' den" about 45 minutes east of Adelaide.

"Lions are always attracted by food. I think they're going to be attracted anyway by the fact there are 20 or 30 people in a cage," she said.

Ms Bensted said well-known Port Lincoln tour operator Rodney Fox got the idea for shark cage diving when he saw lions being fed at the zoo.

"If you've had the opportunity to get close to a lion they're just amazing creatures so to get up close and be able to see a lion or 10 around you, I think, will appeal to people of all ages," she said.

Ms Bensted believed the lions would be "very" interested in the people inside the cage even when they are not being fed.

The zoo also planned to use the people enclosure for close-up experiences with other animals at the zoo, including hyenas.

SA Regional Development Minister Geoff Brock said the new attraction would boost Monarto Zoo visitor numbers above the current annual level of 130,000 annually and inject about $1 million a year into the local economy.

The Federal and South Australian Governments will contribute half of the funding for the tunnel and people cage, which is expected to be operating by the end of this year.

PETA: People cage is 'harassment'

Animal rights group PETA said using a "tunnel to trespass further into these lions' lives is nothing short of harassment".

"Lions do not want humans walking into their den any more than humans want lions wandering into their living rooms," associate director of campaigns Ashley Fruno said.

"PETA is calling on both the state and federal governments to withdraw their funding from this cruel project."

But Zoos SA Life Sciences director Peter Clark dismissed the comments.

"The Predator Experience is all about reversing the traditional zoo experience by putting our visitors in a cage, with our lions free to roam their 10-hectare habitat," he said.

"This project is the first of its kind in Australia and has been developed with modern animal welfare science in mind.

"With 10 hectares to roam, the development is predicated on choice, with the lions choosing where they go."