Sydneysiders should brace themselves for a very wet weekend and the rain will set in for days.

While no warnings for the five-day rain event have been released yet, thunderstorms and heavy rain are expected throughout eastern NSW.

The storm will reach Sydney by Saturday evening.

It will be the second spurt of heavy rain the city has experienced this week after a cold, wet start to the week with a top rainfall of 28mm.

"For inland parts today we're expecting to see the higher rainfall totals and tomorrow along the coastal areas and particularly in between the Illawara and south coast northern parts," said a a spokeswoman at the Bureau of Metereology.