A study by scientists from the University of Sydney has shown that the last remaining populations of the broad-headed snake, found in Bundanoon, are being threatened by encroaching woodland that is destroying their habitat.

Professor Jonathon Webb from the School of Biological Sciences at the University of Sydney said increased plant cover in the Morton National Park is a problem for the endangered species due to increased shading by the vegetation.

"The trend is clear - as the vegetation cover increases, the snakes' available habitat decreases. Our results indicate that active management is required if the nation's most endangered snake species is to be saved from extinction," Dr Webb said.

Using historical and current images of the national park, the research team compared aerial photographs taken in the 1940s and 1970s with satellite images taken in 2006 to ascertain the relative coverage of vegetation.

The study showed that total vegetation cover has increased up to 70 per cent over the 65-year interval.

"The reason for the proliferation of vegetation is not known. In other parts of Australia, vegetation thickening has been attributed to altered Aboriginal fire regimes or to 20th-century climatic change," Dr Shine said.

"Although broad-headed snakes would benefit from controlled burns, the extreme risks of bushfires in Australia must be weighed against the expected gains.

"Alternatively, if controlled burns are deemed too expensive or too dangerous to implement, then foresters might clear overhanging vegetation in areas known to be important to the snakes," he said.