Wednesday's sunny morning quickly took a turn for the worse, as heavy rain and thunderstorms rolled in to drench the region.
Many residents were caught without their brollies by the dramatic change in weather, getting soaked in the 2pm downpour.
The daily temperature peaked about 11am, when the mercury hit 26.1 degrees at Bellambi, before dropping back into the low 20s in the afternoon.
The Bureau of Meteorology issued a storm warning about 5pm, saying the Illawarra and South Coast may experience flash flooding.
However, the inclement weather cleared almost as quickly as it developed, moving on to Newcastle and the Central Coast in the evening.
The rain, which also lashed Sydney, has caused delays for some South Coast commuters who are being forced to wait in long lines just to access the Illawarra line platforms at Central Station.
‘‘We were told water on the lines at Erskinville means there will be delays,’’ Wollongong commuter Daniel Morgan said.
‘‘They said we wouldn't be departing Central for some time due to the storm.’’
Sydney Trains has advised passengers on the Illawarra and South Coast lines to allow extra travel time due to a power supply issue at Sydenham.
They say passengers should listen to station announcements and check indicator boards.