SYDNEY/MELBOURNE - Bushfires burned dangerously out of control on Australia's east coast on Saturday, fanned by high temperatures and strong winds that had firefighters battling to save lives and property, as a change in wind conditions merged several large fire fronts.

By late evening, Victoria had 14 fires rated at emergency or evacuate warning levels, and New South Wales had 11 rated emergency, with more than 150 others burning across the states. New fires had started, and others had broken containment lines.

"There are a number of fires that are coming together - very strong, very large, intense fires that are creating some of these fire-generated thunderstorms," New South Wales Rural Fire Service (RFS) Commissioner Shane Fitzsimmons said at an evening briefing.

"And unfortunately we've still got many hours to go of these elevated and dangerous conditions."

The Bureau of Meteorology (BOM) said conditions were deteriorating rapidly as a gusty southerly wind change pushed up the coast and smoke plumes from the fires triggered storms.

Authorities are worried the fires could turn out to be worse than New Year's Eve, when they burnt massive tracts of bushland and forced thousands of residents and summer holidaymakers to seek refuge on beaches.

In Victoria, Premier Daniel Andrews said while conditions were difficult, the job of firefighters had been made easier by tens of thousands of people following advice to evacuate.

It may be Sunday or later before damage assessments can be made. Prime Minister Scott Morrison put the national death toll from the current fire season, which began in September, at 23. Twelve of those are from this week's fires alone.