A CFA worker at Callignee during the 2009 Victorian bushfires.

A CFA worker at Callignee during the 2009 Victorian bushfires. Photo: Pat Scala

Bushfire risks have been rising steadily for nearly 40 years, according to comprehensive new data that shows for the first time that big firestorms near Australian cities are becoming more likely.

As the nation prepares for an uncertain fire season, the study shows risks increasing most sharply in eastern Australia, based on daily weather data gathered from 38 key sites around the country between 1973 and 2010, according to the the study published in the International Journal of Climatology. None of the sites were safer from severe fire conditions than they were a generation ago.

The study is based on the forest fire danger index, or FFDI, a points system that underpins the standard low to catastrophic bushfire rating system.

''Fire weather, as depicted by the FFDI, has increased across much of Australia since 1973,'' said the report, produced by researchers at the Climate Change Research Centre at the University of NSW.

''The largest absolute changes occur in the hot, arid interior of the continent, although some of the largest proportional increases occurred in coastal areas, where average annual cumulative FFDI is relatively low - Melbourne and Adelaide recorded increases of 49 per cent or more over the duration of the record.''

The FFDI rating system involves tallying the data on rainfall, evaporation, heat, wind speed and humidity to arrive at a number that signifies the degree of fire risk on any given day.

Since 1970, the average temperature across Australia has increased 0.7 degrees Celsius, but in some regions that average temperature is two degrees warmer than it was 40 years ago.

The trend across nearly the whole country was for longer fire seasons, and more fires in spring, though not necessarily more fires in summer.

As the likelihood of a heat-bringing El Nino cycle fades for this year, the nation is preparing for a predicted average fire season, albeit one with a large build-up of fuel from the past two years, which were unusually wet.