Evidence began on Monday in the Royal Commission into National Natural Disaster Arrangements hearings, which will look into the cause and response to bushfires including those that devastated the Border over summer.
Bureau of Meteorology head of climate monitoring Karl Braganza told the hearing Southern NSW and Eastern Victoria had experienced significant fire events every season in the past 17 years.
"This isn't a one-off event we're looking at here," he said.
"The frequency of these events, if we look at the historical record, seems to be increasing.
"These large fire events, when you look back over the 20th and 19 century, were not as frequent as this century."
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The Border's worst fires in the past had traditionally been in February and March, but Dr Braganza said the BOM had predicted the fires now known as "Black Summer" would start much earlier.
"Across almost all of Australia we're seeing a longer fire season with more fire danger days during that season and the severity of the worst fire danger days is becoming more severe," he said.
"The fire season is extending and that is probably most significant in spring.
"We're getting early season heatwaves."
Royal Commission chair Mark Binskin said as hearings began, he understood communities were still grieving due to the loss of lives, homes, livestock and forest.