Flash flooding that hit parts of NSW on Wednesday is unlikely to dry up on Thursday with the state to cop another drenching.

A low pressure system that hammered northern NSW is expected to move south to Sydney, the Illawarra, the Hunter and the Blue Mountains regions, continuing the long run of rainy days.

But the end is in sight for Sydney with the sun expected to finally peek through on Sunday.

"Today is probably the last of the significantly wet days in Sydney but we will still see quite a few showers on Friday and Saturday then, from Sunday, it will ease significantly," said Rob Sharpe, meteorologist from Weatherzone.