It's conservation by numbers, or - as one headline writer put it - "survival of the cheapest". In a country with one of the world's highest extinction rates, scientists are using a mathematical equation to determine which species should be saved and which let go.

The approach, adopted by the New South Wales Government this month, reflects the practical impossibility of rescuing all of Australia's threatened plants and animals from the brink. Proponents call it a more efficient and effective way of targeting limited resources.

Critics, though, are uneasy about accepting some extinctions as inevitable, and say governments should be devoting more resources to the fight to preserve wildlife. They also fear that a "triage" system - used by hospital emergency departments to prioritise treatment - will favour "charismatic" species such as the koala over an obscure bat or stick insect.

The mathematical formula, devised by Queensland University scientists, involves multiplying the benefit (in dollar terms) of an endangered species surviving by the likelihood of a conservation programme succeeding, then dividing that figure by the programme's cost.

Species which score highly should be prioritised in terms of funding, according to the strategy, which has been adopted by New Zealand's Department of Conservation and is being considered Australia-wide.

Professor Hugh Possingham, director of Queensland University's Centre for Excellence in Environmental Decisions, believes many more plants and animals could be saved if a rational mathematical approach was adopted. While it might sound mercenary, and means certain species being written off, it's not any different from running any other business, Possingham told Fairfax Media. He added: "Everybody in the world, every day, is doing a cost-effective analysis."

Under the algorithm, species such as the masked owl and yellow-spotted bell frog would be prioritised by New South Wales, with the koala, which is worth an estimated A$1 million ($1.1 million) a year in tourism dollars. Those given a lower priority would include the purple-crowned lorikeet, which is rare in NSW but found across southern Australia.

More than 100 native plants and animals have vanished since European colonisation. The country has the world's worst record for mammal conservation, accounting for nearly one-third of mammal extinctions over the past 200 years.

Rather than directing funds towards species at greatest risk of extinction, the mathematical approach targets those with the best chance of survival. Calculation of "benefit" is based not only on economic value, but on factors such as the role of a species in an ecosystem. Flying foxes, for instance, perform a vital function because they disperse fruit and pollinate trees. Critics such as Belinda Fairbrother, NSW campaign manager for the Wilderness Society, say Government funding for conservation is grossly inadequate.

Possingham acknowledges that many more species could be saved if funding was increased. Writing in The Conversation, an online academic forum, he noted that about A$3 million is spent each year on conserving threatened Australian birds - a sum equivalent to less than 1 per cent of the weekly defence budget. If that sum were tripled, the number of bird extinctions over the next 80 years could be reduced to almost zero, he wrote, and the number of threatened species cut by about 15 per cent.

New Zealand was the first country to apply the algorithm, adopting it about five years ago. Richard Maloney, a senior DoC scientist who has been advising the NSW Government, said 300 threatened species had been identified as most likely to benefit from conservation measures.

Early results indicated a fantastic improvement, he told Fairfax.