Sydney is expected to be drenched in more rain on Wednesday than originally forecast, with up to 90 millimetres possibly falling across the city in the coming day.
It will be the start of a soggy three days across the harbour city, with the intensity of the rainfall expected to peak towards the end of the evening commute on Wednesday.
The Bureau of Meteorology had originally predicted falls of between 25-70 millimetres across the city on Wednesday, but that rainfall total was later increased to between 40-90 millimetres.
"There are showers about the Sydney region now," bureau forecaster Katarina Kovacevic said early on Wednesday morning.
"As the day continues, we will see those showers tend more to rain, and then by this evening, we're anticipating that rain might actually tend heavy at times."
Ms Kovacevic said heavy rain could coincide with the end of the daily commute, about 7pm on Wednesday.
The rain was being driven by a low pressure system that was expected to cross the central part of the NSW coast on Wednesday evening.
It will bring gusty winds and heavy rainfall to parts on the coast, before it is expected to move away into the Tasman Sea on Thursday morning. Sydney can expect south-easterly winds of between 15-20 km/h throughout the day, turning southerly 20-30 km/h during the afternoon and evening.
The bureau issued a Flood Watch for potential minor flooding in the Macintyre River Valley in northern NSW, including the Severn River, from Wednesday onwards.
"This Flood Watch means that people living or working along rivers and streams must monitor the latest weather forecasts and warnings and be ready to move to higher ground should flooding develop," the bureau warned.
Ms Kovacevic said the rainfall should ease in Sydney once the the low pressure system moved away from the coast on Thursday morning.
Sydney is forecast to receive between 2-10 millimetres of rain on Thursday, and between 3-10 millimetres of fain on Friday.
A top of 15 degrees is forecast in Sydney on Wednesday, and 16 on both Thursday and Friday.
Weatherzone meteorologist Brett Dutschke said Sydney was presently experiencing its wettest year-to-date in 18 years.
A total of 1138 millimetres of rain had fallen across the city from January 1 up to Wednesday morning. That figure stood at 1433mm on the same date in 1998.
Weatherzone is owned by Fairfax Media, publisher of this website.