There's little sign of temperatures taking a dive as parts of NSW and Victoria roast again. Credit:Jessica Hromas

South-eastern Australia's big autumn heatwave has again pushed temperatures to record levels with Sydney threatening to set a new high mark of April for the second time this week - with another chance on Friday.

The Harbour City hit 34.6 degrees by mid-afternoon Thursday. That level has only been exceeded once in April in the past 159 years - on Monday, when the mercury touched 35.2 degrees.

The Bureau of Meteorology forecast Sydney's top on Friday will again climb to 35 degrees, with another balmly night with a minimum of 22 degrees in between.

The average maximum temperature so far this month in Sydney is about 28.5 degrees, or six degrees above the long-run April average. A typical January day in the city is about 26 degrees.