'Potentially life threatening' Rain and flooding taking hold in New South Wales

Intense rainfall is developing across parts of New South Wales with forecasters predicting it could lead to "life-threatening flash flooding".

Sydney has seen rainfall for much of the past week but the NSW Bureau of Meteorology (BoM) says Saturday's downpour will be "substantially heavier".

"We might see quite significant flash flooding and we've got a flood watch current for both the Nepean and Hawkesbury rivers," bureau flood operations manager Justin Robinson is quoted as saying by 7 News.

It follows intense rain across the mid-north coast on Thursday and Friday which caused flood rescues and evacuations. 

The rainfall across the mid-north coast saw dozens of residents trapped by rising water, and according to 7NEWS, rescuers completed up to 57 flood rescues and responded to 1300 emergency calls. 

Flooding has also resulted in several road closures, including one where a road was washed away in Port Stephens. 

Major flooding and Road closures forcing towns such as Macksville and Bowraville to be cut off and left isolated. 

People living in low-lying areas of Port Macquarie also had to evacuate on Friday night with 7NEWS reporting a "significant" number of properties are expected to be flooded. 

On Saturday morning BoM issued a severe weather warning and a hazardous surf warning for more than half of the NSW coastline.

It said damaging winds and gusts exceeding 90km/h are expected to develop, causing wave heights of five metres and leading to possible coastline erosion. 

"We're likely to see a spill from Warragamba Dam and that will then impact those communities downstream," said Robinson.

Sydney's rainfall won't be as intense as the rain that occurred across the mid-north coast, with the worst of the downpour passing - yet emergency services are warning road and surf will continue to be dangerous. They are asking motorists and boaties to stay at home. 

The rain is set to extend into the next week.