A severe weather warning has been issued for much of the NSW coastline as a wet weather system hits the state delivering an estimated 200 millimetres of rain in parts, which is expected to cause flash flooding.
The warning, issued on Friday by the Bureau of Meteorology, says a low-pressure trough is deepening with rain to become more frequent and intense throughout Friday night.
Heavy rain and thunderstorms may lead to flash flooding on the mid north coast and far northern parts of the Hunter region over the long weekend.
The BOM is predicting 24-hour rainfall totals between 120mm and 200mm at the system's centre in Yamba.
The heavy rain is likely to persist over the weekend with the coastline from Coffs Harbour up to Byron Bay expecting up to 100mm of rain on Sunday as well.
"We're expecting to see high rainfall totals when the system crosses the coast," bureau meteorologist Rosemary Barr told AAP on Friday.
"Areas of the coast with sharp topography will have a higher chance of heavier rainfalls as the system pushes upward."
Ms Barr said the weather system was not an east coast low, the type of damaging storm that washed the Pasha Bulker ashore in Newcastle 10 years ago and claimed lives in the Hunter Valley in 2015.
"It's currently a coastal trough, which is similar but not as intense when it comes to wind speed," she said.
"Waves are also linked to wind speed so the surf isn't within our warning criteria."
The BOM said some of the locations in the path of the system are Lismore, Grafton, Coffs Harbour, Port Macquarie, Taree and Dorrigo.
Rain is expected to ease in the southern parts of the mid north coast and far northern parts of the Hunter by Saturday morning.
