Days of rain lie ahead for parts of eastern NSW with significant flood impacts, damaging winds and large waves.
More than 20 flood rescues have already been carried out in northern NSW with at least one person taken to hospital after being rescued from their flood-stricken vehicle.
The State Emergency Service said it responded to more than 350 calls for help overnight on Sunday.
Of those, most were made from the state's northern region with 60 coming from Sydney relating to fallen trees, blocked roads and property damage.
A severe weather warning for heavy rainfall and damaging winds stretches from the state's mid-north coast at Kempsey to the Central Coast.
Isolated falls of up to 120mm over six hours are possible, leading to flood watch warnings across the same region.
"We're already seeing those coastal catchments respond quite quickly to rainfall over the weekend," NSW SES operational media co-ordinator Emily Barton said.
"With the forecast heavy rainfall over the next few days, we're anticipating renewed rises and many catchments to reach minor to moderate flooding.
"SES has pre-positioned flood rescue teams, high clearance vehicles and aviation assets throughout the high-risk areas, but we're urging the community to stay alert."
Bureau of Meteorology senior meteorologist Angus Hines said a band of rain that formed overnight into Monday was shifting north to the central and northern coastal regions.
Mr Hines called the extended nature of the wet weather "significant" with rain expected to fall in the same region for some days.
"We're looking at rain for most of the day today, then again tomorrow and then Wednesday, and potentially out towards Thursday," he said.
"So it's not a short, sharp rain event ... it's a long stretch of maybe moderate rainfall at any one time, but several days in a row of moderate rain will give you big numbers so we could see well and truly in excess of 200mm.
"Some places will probably get closer to 300mm or 400mm over the course of the entire week."
Mr Hines said the regions with the highest risk of damaging winds and heavy rainfall were the central, Hunter and mid-north coasts, along with parts of the central and northern ranges.
Dangerous beach conditions are also possible from Sydney's southern beaches to Seal Rocks with waves up to 5m possible.
A damaging surf warning has been issued for the coastline stretching from Gosford to Coffs Harbour.
Gusts of around 100km/h are expected over the Hunter and mid-north coast regions.
Widespread minor to moderate flooding is also expected through rivers north of Newcastle and south of Coffs Harbour over the coming days.
Wyee, in the Central Coast region, had the state's highest rain total between 9am Sunday and 6am Monday with 121mm.
Nearby, Dora Creek had 117mm.
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