The Weather Service warned that “large flooding” on the north-central coast of New South Wales has caused a major river to rise to its highest levels in nearly 100 years.

Dean Narramore's Meteorologists Bureau predicts rainfall in parts of state north of Forster on Wednesday is 50mm to 150mm, with isolated ones likely exceeding 200m.

Naramore said some areas were flooded with rainwater from 300mm to 400mm - the town of Tari is larger - causing extensive mountain flooding and major river flooding.

He said flooding may not ease until later this week as slow-moving weather systems continue to dump rainfall in the area.

"(Weather System) swings north today and then likely will go back to the south, unfortunately tomorrow and finally clear from the Central North coast," he said.

“There were two days of rain on the way, and then a lot of floods.”

Coffs Harbour, Macquarie Harbour, Tari, Kempsey, Sortle, Dorigo, Barrington Tops, Winham and Yarrowitch are all lined up to rain.

Locals around the Manning River were warned to evacuate Tuesday night as the river crossed major flood levels.

The NSW Emergency Service said the river was at a "never seen" level in the tower by Wednesday morning, surpassing the 1929 six-meter record.

In the past 24 hours, SES responded to 892 calls, including 130 flood rescues, most of which were in Tari, Wintham and Glenthurn.

The rescue includes several cars driven into the flood, and as the night progresses, many reported rising houses on the water, and some needed to seek shelter on the roof, SES said.

“Unfortunately, we have…have a lot of calls for help in areas where evacuation warnings and we are still evaluating how best to get into these locations.”

In the state’s Hunter area, flooding is one of the areas for flood surveillance, where residents warn residents to monitor forecasts and rainfall and prepare to move to higher ground.

For Ray, a cleaner at Bulahdelah's Plow Inn Hotel, the emerging river has caused losses.

“My boathouse is gone,” he said. "I've been traveling through the docks (Tuesday) mornings and that's not there. It's the icing on the cake (the weather has been coming out for the last few weeks."