Australian officials say record floodwaters on the country's east coast have left one person dead and three others missing, as more heavy rain is forecast in the area.

Five people were confirmed killed on Friday in record flooding in northern New South Wales with 50,000 people cut off, thousands of homes and businesses without power and cattle herds wiped out.

Emergency services discovered the body of an 80-year-old man on his property in the Mid North Coast region of New South Wales on Friday, bringing the confirmed death toll from the devastating flooding on Australia's east coast to five.