Fears that most of the Earth's species will become extinct before they have even been discovered by science are "alarmist", according to an international study released on Friday.

Researchers set out to examine estimates that there were 100 million species globally and they were dying out at a rate of five percent every decade, meaning many would disappear before scientists had a chance to discover them.

Writing in the journal Science, the researchers from New Zealand, Australia and Britain said the estimates were based on a massive over-estimation of how many species were still unknown.

They said about 1.5 million species of animals and plants had already been catalogued and statistical modelling showed the total number in existence was closer to five million than 100 million.

The study, released Friday, also put extinction rates at less than one percent a decade, one-fifth the level of previous estimates.

"Our findings are potentially good news for the conservation of global biodiversity," lead author Mark Costello from the University of Auckland said.

"Over-estimates of the number of species on Earth are self-defeating because they can make attempts to discover and conserve biodiversity appear to be hopeless. Our work suggests this is far from the case."

He said the research raised the prospect that all of Earth's species could be identified within the next 50 years, particularly since the number of taxonomists, scientists who describe new species, was increasing.

"Naming a species gives formal recognition to its existence, making its conservation far easier," Costello said.

The paper conceded that Earth was in the midst of a "human-caused mass extinction phase" but reached more optimistic conclusions on biodiversity than other researchers, such as those at the California Academy of Sciences.

In 2011, the academy said: "Despite intensive efforts to document life on Earth, scientists estimate that more than 90 percent of the species on this planet have yet to be discovered.

"In the face of large-scale habitat loss and degradation, many of these species are disappearing before we even know they exist."

Costello and his colleagues said any meeting of biologists or conservationists was "hardly complete" without similar worries being raised.

But they said the development of science in biological hotspots such as Asia and South America meant more researchers than ever were working to identify new species.

"Some people despair that most species will go extinct before they are discovered," the study said.

"However, such worries result from over-estimates of how many species may exist, beliefs that the expertise to describe species is decreasing and alarmist estimates of extinction rates."