Australia's most populous state of New South Wales (NSW) braced for its worst flooding in decadeson Sunday, with authorities issuing warnings of a potentially "life-threatening" situation.

Sixteen areas of NSW, primarily in the north, were declared "disaster ridden," NSW Police and Emergency Services Minister David Elliott said during a press conference, adding that emergency services responded to 640 calls for help, including 66 for flood rescues.

Residents in flood-hit areas west of Sydney were evacuated overnight after heavy rainfall caused its largest dam to overflow.

NSW Premier Gladys Berejiklian said the region was experiencing a "one-in-100-year event" and that a national disaster had been declared, adding that some locations in Western Sydney recorded more than 300 millimeters (11.8 inches) of rain since Friday morning, breaking records.

"Whilst we don't think things will worsen on the Mid North Coast, definitely conditions will continue, so the rainfall will continue across the parts that have already been affected," Berejiklian told journalists.

So far there are no reports of deaths.

Bodyboarder 'missing in waters'

The NSW Bureau of Meteorology (BoM) warned that the Nepean Valley, which incorporates the major urban center of Penrith – part of Greater Western Sydney –  was set to experience its worst flooding in 50 years as overflowing of the Warragamba Dam caused river levels to rise along the Nepean and Hawkesbury rivers.

Severe flooding struck parts of Port Macquarie, some 400 kilometres north of Sydney, including several nearby towns.

Local television showed footage of a three-bedroom cottage in the town of Mondrook being swept away from its foundation by floodwater.  

"It is one of the biggest floods we are likely to see for a very long time," Bureau of Meteorology flood operations manager Justin Robinson said.

NSW police said they were looking for a bodyboarder believed to be "missing in waters" at Coffs Harbour, some 430 kilometers north of Sydney.

Further evacuations expected

So far, 13 evacuation centers have been opened across NSW. More evacuations are expected as the bad weather is forecast to last into the middle of the week.

A severe weather warning was also in place just west of the capital Canberra.

Deputy Commissioner Daniel Austin told Australia's Channel Nine's Today Show on Sunday morning that emergency services were planning their operations "well past Easter."

"Just because the rain stops it doesn't mean that the rivers go back to normal levels straight away," he said.

mvb/aw (AP, AFP, dpa)