Sydney will see temperatures exceeding 30 degrees on Thursday, before a "classic southerly buster" brings a 10-degree drop for the weekend.
Temperatures will reach 30 degrees in the city's inner suburbs and 31 at Penrith and Liverpool, as a northerly wind moves in from the north-west of the state.
However, the mercury will drop suddenly about 5pm or 6pm on Thursday evening, as a southerly change moves up from the South Coast.
"It's a classic Sydney southerly buster," said Bureau of Meteorology forecaster Helen Kirkup. "When we move into spring we do get these real switches in the weather as we settle into summer conditions."
A maximum temperature of 20 degrees and a possible shower is forecast for Friday, although Ms Kirkup said it is more likely to be windy than wet.
A strong wind warning is in place for coastal areas in Sydney, the Illawarra and the South Coast.
The cooler temperatures will remain in the city over the weekend, with maximums of 22 and 23 forecast for Saturday and Sunday, respectively.
Warmer weather will return on Monday, when a high of 27 degrees is expected.
Scheduled hazard reduction burns will go ahead around Sydney on Thursday, despite the high temperatures.
RFS spokesperson Ben Shepherd warned Sydneysiders would "see and smell that smoke", but it was important the necessary work was done "now" ahead of higher than usual rain activity forecast for spring.
"It is windy and we do consider that, but we also have a southerly blowing in with chance of showers," he said, noting some fire agencies conducting the work had changed their lighting patterns due to the weather.
Scheduled burns on Sydney's northern beaches were postponed due to moisture levels following rain activity earlier in the week.