His uncovering of a fossilised leaf embedded in the rock opened the door to an extraordinary site that's accelerating our understanding of prehistoric life in Australia.

"Many of the fossils that we are finding are new to science and include trapdoor spiders, giant cicadas, wasps and a variety of fish," Australian Museum and University of NSW palaeontologist Dr Matthew McCurry said.

"Until now it has been difficult to tell what these ancient ecosystems were like, but the level of preservation at this new fossil site means that even small fragile organisms like insects turned into well-preserved fossils."

Over the last three years, Dr McCurry, colleague Dr Michael Frese and a team of researchers have been secretly excavating and analysing McGraths Flat near Gulgong on the Central Tablelands, discovering thousands of specimens including rainforest plants, spiders, fruiting bodies, fish and a bird feather.

Among the insects uncovered were wasps, ants, cicadas, mayflies, beetles, flies and assassin bugs.

The quality of the fossils means interactions between species can also be determined.

The stomach contents of fish have been preserved, allowing the researchers to determine what was on the menu 15 million years ago.

"We have also found examples of pollen preserved on the bodies of insects so we can tell which species were pollinating which plants," Dr Frese said.

The fossils are of such exceptional quality, McGraths Flat has joined the handful of Lagerstatte sites in Australia.

The dig itself lies on the NSW Central Tablelands, about 25 kilometres northeast of Gulgong, a 19th-century gold rush town.

It's also a short drive from another important Australian fossil site, Jurassic-era Talbragar.

But unlike its older cousin, McGraths Flat opens a window to the Miocene epoch, a time of immense change in Australia when the continent was drifting northwards.

When the Miocene began 23 million years ago, Australia was rich with diverse plant and animal life, only for an abrupt change in climate to cause widespread extinctions about 14 million years ago.

The scientists believe the discovery provides an important case study for figuring out which species can adapt to a changing environment, and which go extinct.

"The McGraths Flat plant fossils give us a window into the vegetation and ecosystems of a warmer world, one that we are likely to experience in the future," Royal Botanic Gardens Victoria director of science Professor David Cantrill said.

"The preservation of the plant fossils is unique and provides important insights into a time period for which the fossil record in Australia is rather poor."

Dr McCurry thinks the process that turned the organisms into fossils is key to why they are so well preserved.

"Our analyses suggest that the fossils formed when iron-rich groundwaters drained into a billabong, and that a precipitation of iron minerals encased organisms that were living in or fell into the water," he said.

Dr McCurry and his colleagues' findings were published on Saturday in Science Advances.

The fossils will be stored in the Australian Museum's palaeontology collection to allow further study.

The study was partially funded by a descendant of Robert Etheridge, the museum's first palaeontologist.

Australian Associated Press