Parts of the NSW north coast are on flood watch with a deep low-pressure system expected to cross the coast on Friday, delivering torrential rain and destructive winds to some areas and drenching Sydney.
Widespread rainfalls of 50 millimetres are forecast across Sydney, beginning with showers on Friday morning that will steadily increase throughout the day and become heavy rain overnight and into Saturday.
Areas of the NSW north coast that are still mopping up after ex-tropical cyclone Oswald last month are expected to be hit worse, with widespread falls of 100-200 millimetres across for the area.
The heaviest rain - possibly in excess of 300 millimetres - is expected to fall between Grafton and Port Macquarie.
"Places like Coffs Harbour and Kempsey could get a drenching," said Weatherzone meteorologist Melissa MacKellar.
Richie Williamson, Mayor of the Clarency Valley Shire which includes Grafton, said the region was being buffeted by 100 km/hour winds and heavy downpours amid "a very, very significant event."
"We had a record-breaking flood just three weeks ago and we are faced with the very real situation of another flood," Mr Williamson said.
Weatherzone's Dr MacKellar said the low-pressure system was located off the north coast on Friday morning, and was expected to cross the coast somewhere north of Coffs Harbour during the afternoon.
The Bureau of Meteorology has issued a flood watch for NSW coastal river valleys from the Queensland border to the Hunter Valley, and a severe weather warning for destructive winds, damaging winds, heavy rain and damaging surf for people in the Northern Rivers, Mid North Coast, Hunter and Northern Tablelands forecast districts.
Waver rider buoys off the coast of Byron Bay and Coffs Harbour recorded waves as high as nine metres on Thursday night. Off Sydney's coast, seas of between 4-5 metres are expected to cause coastal erosion.
Damaging winds of about 60km/h, with gusts up to 100km/h, are forecast over the Northern Rivers and Mid North Coast forecast districts on Friday morning, with coastal areas most likely to be affected.
Locally destructive winds with peak gusts of 125 km/h are possible between Ballina and Kempsey. Damaging winds gusts of 100 km/h may affect eastern parts of the Northern Tablelands on Friday afternoon, and could extend to northern parts of the Hunter coast this evening.
Dr MacKellar said the wind would also pick up in Sydney throughout Friday, with south-easterly winds increasing to 35-55 km/h in the evening and some gusts reaching 80-90km/h.
"The conditions will ease on Sunday in Sydney," Dr MacKellar said.
"Basically during the next week there are going to be onshore winds and there will be showers each day and temperatures in the mid 20s, pretty close to average. As the low weakens there will be a trough lingering over much of NSW and there will be a good chance of storms in Sydney on Monday and Tuesday."
A maximum temperature of 25 degrees in forecast in Sydney for Friday, rising to 26 degrees on Saturday and 29 degrees on Sunday.
Grafton and surrounding districts are protected from floods by a system of levees stretching 108 kilometres, some of which were damaged when floodwaters came close to breaching the defences three weeks ago.
Clarency Valley Shire staff have door-knocked some rural areas where the recent floods had caused some damage to the levees, warning residents to be prepared to evacuate if needed.
"Council staff are constantly monitoring the levee systems and at the moment I am confident that the main levee systems around Grafton are sound," Mr Williamson said.
With Peter Hannam
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