Sydney has set record warmth for April as much of south-eastern Australia basks in conditions typical of mid-summer, with parts of the city reaching "severe" fire danger.

The mercury in Sydney had reached 35.4 degrees just before 2.30 pm, exceeding the April record notched just two years ago of 34.2 degrees by more than a degree.

Other parts of the city have notched at least late-season heat records.

Richmond's 35.9 degrees was also well above the previous record of 33.4 degrees for this far into April, according to Blair Trewin, senior climatologist at the Bureau of Meteorology.