IT IS not just the long summer days and the scantily clad bathers on Bondi Beach that keep Australia hot. It is also the radioactivity beneath the continent's red dust.

Australian scientists have achieved a world first by compiling a highly detailed "radiometric" map of the continent's key natural radioactive elements.

Produced by the geological agency Geoscience Australia, with the help of state and territory authorities, the colour-coded map shows areas rich in uranium as blue. Radioactive potassium is red, while thorium is green. Bone-coloured regions are rich in all three.

Most of the country is brightly shaded, but Australians did not have to worry about radioactivity beneath their feet, Geoscience Australia's James Johnson said yesterday.

"We are detecting very low levels of these elements. It's a very sensitive method. There are not rocks you can't walk on," Dr Johnson said.

The map had been "seamlessly stitched together" from data collected over 40 years by aircraft carrying instruments able to detect tell-tale gamma radiation.

By global standards, Dr Johnson said, "our patch of the Earth's crust" has a high proportion of granite - often a good source of radioactive minerals.

"Australia has the world's largest global resource of uranium, something like 38 per cent," he said.

Regions with relatively high levels of radioactivity could be candidates for "hot rock" thermal power stations, which would be driven by water heated by natural underground energy.

The radiometric map would be available free to anyone who wanted it, Dr Johnson said,

"It will be a dramatic aid to mineral exploration."

He noted that most of Australia's geological wealth had been discovered by prospectors studying mineral outcrops "sticking out of the ground".

Such continental maps allowed geologists to look beneath the surface of the entire country.

"There is untold opportunity beneath the red sand," he said.

Sydney is one of several regions not covered by the radiation survey. Not only was it unsuitable for mining, it was not ideal for low-flying aircraft, Dr Johnson said.