They're slightly smaller than Australia's blue-tongue lizards, are flower-eating vegetarians and remain one of the world's most puzzling evolutionary enigmas.

A new species of Pacific iguana Brachylophus bulabula named after the Fijian word for ''hello'', or in this case, ''hello, hello'' has been discovered by a team of scientists led by Australian National University biologist Professor Scott Keogh.

''It's a pretty exciting find. There were previously thought to be only two living species of these iconic and very beautiful iguanas until we looked at the genetic data in the laboratory. After we did the molecular work, we did a morphological analysis to establish if there were any obvious physical differences between the three species,'' he said.

The third Pacific iguana species does have several subtle differences that distinguish it from its relatives a different coloured nose (yellow) and eyes (reddish-brown), as well as a mid-range body size.

The other two species are the Fijian crested iguana (Brachylophus vitiensis) and Fijian banded iguana (Brachylophus fasciatus). A common name for the ''hello, hello'' iguana is yet to be decided.

News of the discovery was published this week in a special Charles Darwin commemorative edition of the Royal Society journal, paying tribute to the great evolutionary biologist's scientific contribution to the Pacific region.

The new iguana, which is found in the central regions of Fiji, was discovered during an analysis of mitochondrial DNA samples taken from 61 iguanas on 13 islands across the Fijian archipelago.

The research team wasn't looking for a new species, but trying to solve the mystery of how the ancestors of Fiji's iguanas arrived on islands in the middle of the Pacific Ocean some 13 million years ago. They're believed to be descendants of the green iguanas of south and central America.

''Their closest relatives are 8000km away, so it seems they must have rafted over from there somehow. How they got here is a famous mystery that has puzzled scientists for a long time,'' Professor Keogh said.

But the new species is already headed for the critically endangered list, with survival of all Pacific iguanas under threat from predation by feral cats and mongooses and loss of food and habitat from overgrazing by goats.

''There were two bigger iguana species but they were both eaten to extinction after people arrived nearly 3000 years ago,'' Professor Keogh said. Rosslyn Beeby