Australia has issued a “catastrophic" fire warning across Sydney, the country's largest city, as firefighters battle a rapidly worsening bushfire crisis.

Fires have killed three people and destroyed more than 150 homes since Friday. There were still more than 80 fires burning across New South Wales on Sunday, despite the efforts of more than 1,300 firefighters supported by 70 fire-fighting planes.

The New South Wales Rural Fire Service (RFS) said Greater Sydney, the Hunter, Blue Mountains and Central Coast should brace for "catastrophic" fire conditions in the coming days.

It is the first time Greater Sydney has received this level of warning since the ratings system was introduced a decade ago.

The RFS and the Bureau of Meteorology warned that while Monday may see an easing in weather conditions, the forecast for Tuesday is dire.

The RFS said that “large areas” of New South Wales are “expected to see severe or extreme fire danger”.