The Bureau of Meteorology (BoM) is forecasting widespread and intensely torrential rain and strong winds across the region today (Friday) and into the weekend due to a deepening trough sitting over NSW.

Flooding is possible and flood warnings have been upgraded for the Mid North Coast through to the Hunter.

Zone commander for the NSW State Emergency Service (SES), chief superintendent Stephen Patterson, said the BoM has forecast widespread rainfall of between 50mm to 200mm across much of the Mid North Coast with isolated dangerous falls of up to 300mm.

If the weather system intensifies, widespread moderate flooding could develop across the Orara, Coffs Coast, Bellinger, Kalang, Nambucca, Hastings, Manning, Gloucester, Wallis Lake and Myall river catchments.

Since 9am Thursday, the heaviest falls had been recorded at Kempsey with 185mm. Bellingen has had about 150mm.

"Of particular interest is the Bellinger and Kalang River catchment on the Mid North Coast which has been upgraded to moderate to major flooding in the Flood Watch, while the Paterson and Williams catchments in the Hunter have also been upgraded to moderate to major flooding," Mr Patterson said.

"Moderate flooding is forecast for the Nambucca and Hastings Rivers, while minor to moderate flooding is forecast for the Orara, Camden Haven, Manning, Gloucester, Wollombi Brook, Lower Hunter River and Wallis Lake.

"With many areas already saturated by a wet storm season so far, we can expect to see some riverine flooding along the coast in the coming days, with the potential for flash flooding given the predicted amounts of rain.

"Knowing this, we are asking people to stay off the roads where possible and the NSW SES is also warning vulnerable businesses and facilities like schools and nursing homes to know their flood risks and have plans in place to prepare for the forecast rain.

"And if you come across floodwater while driving, never drive, walk, or ride through it - if the road is flooded, forget it.

"It doesn't matter how experienced a driver you think you are, or whether you have a big car, floodwater can wash away roads that once lay underneath, and even contain chemicals and debris."

Large swell is expected to build across the NSW coastline over the coming days and peak on Saturday, March 20, then slowly drop off over Sunday and into next week.

The conditions will pose a heightened risk to rock fishers on exposed and south facing rocks and platforms.

Surf Life Saving NSW director of lifesaving, Joel Wiseman said people shouldn't put their life on the line and are encouraged to avoid rock fishing on the open coast in the coming few days.

"Recent rock fishing tragedies have highlighted the dangers of this activity and we are urging people to take their safety seriously," Mr Wiseman said.

Marine Rescue is urging boaters to secure their boats' moorings and storm covers and take all possible steps to prevent vessels from breaking free and being washed by floodwaters on to rocks, the shoreline or nearby boats.

Boaters should stay home rather than heading out on the water until the adverse conditions abate. Coastal bars will become extremely hazardous, with conditions offshore becoming increasingly dangerous with heavy rain and poor visibility, big seas, swell and strong winds.