New Zealand has set itself an environmental goal so ambitious it has been compared to putting a man on the moon - ridding the nation of every last rat, opossum and stoat.

The idea is to give a second chance to the distinctive birds that once ruled this South Pacific nation.

When New Zealand split away from the supercontinent Gondwanaland 85 million years ago, predatory mammals had not evolved. That allowed birds to thrive, and some gave up flight altogether to strut about the forest floor.

Then humans arrived, bringing predators with them. Rats stowed away on ships, and settlers introduced opossums for the fur trade and stoats to control rabbits.

When it comes to heritage, he said, England has its Stonehenge, China its Great Wall, France its Lascaux cave paintings. What makes New Zealand unique, he asked? Its birds.

The pests destroyed forest habitats and feasted on the birds and their eggs. More than 40 species of bird died out and many others remain threatened, including the symbolic kiwi.

Scientists are talking about the new mission in military terms: choking off pests on peninsulas and then advancing the front lines from there; developing new traps and genetic weapons; winning the hearts and minds of children and farmers alike.

Momentum began growing five years ago when the nation's leading scientist, Sir Paul Callaghan, delivered an impassioned speech. When it comes to heritage, he said, England has its Stonehenge, China its Great Wall, France its Lascaux cave paintings. What makes New Zealand unique, he asked? Its birds.

Sir Paul was suffering from advanced cancer and could barely stand, but for over an hour he outlined his predator-free vision, saying how growing up he was inspired by efforts to reach the moon and how saving the birds could become New Zealand's own Apollo programme. He died a month later, but the vision grew.

Nine months ago, it became official government policy. Then-prime minister John Key announced a goal to wipe out the nuisance animals by 2050, calling it the "most ambitious conservation project attempted anywhere in the world".

The goal has been embraced by many, although even its strongest supporters say it will require scientific breakthroughs. Some critics argue the plan should also have targeted feral cats and worry mice numbers might explode if rats disappear. Others say the effort is underfunded and overly ambitious.

"It's a fantasy science fiction," says a wildlife biologist at the Victoria University of Wellington, Wayne Linklater. "And it really is seriously distracting us from some really big changes and improvements we can make in biodiversity and the environment now."

The number of pests in New Zealand is many times larger than the human population of nearly five million. Opossum numbers in 2009 were estimated at 30 million. Scientists cannot hazard a guess at how many rats there are because their numbers fluctuate wildly.

So far, the government has committed only a few tens of millions of dollars toward the project, which is estimated to cost billions. Officials say more money will come from local authorities and philanthropists.