Parts of eastern Australia have been declared a natural disaster zone after a “once in a century” flood stranded thousands of people in regional towns, and heavy rain also forced the closure of Sydney’s most popular beaches.

More than 2,000 people were cut off in towns along the mid-north coast of New South Wales after over 450mm (18in) of rain fell in the past 24 hours, flooding towns along the Bellinger River in the Coffs Harbour area.

The main highway through Coffs Harbour — a big NSW city about 350 miles north of Sydney – was turned into a river and hundreds of homes and businesses were submerged.

Hundreds of people were evacuated as the nearby rivers became swollen to near record levels, turning the racecourse into a lake. Students at one school were forced to bunk down in classrooms when they realised they could not make it home, while other people sought refuge in restaurants as the flooding cut off most of the main roads in the area.

More than 800 residents spent last night in evacuation centres before they were relocated this morning.

Hundreds of emergency services volunteers, police and fire fighters continue to work in the area where 100 people had to be rescued overnight.

In the town of Bellingen, still cut off from main roads, 1,600 people remain isolated without power or water.

Steve Whan, NSW Emergency Services Minister, and Murray Kear, and Emergency Services director-general, toured the flood-ravaged region early today and declared the area a natural disaster zone, enabling residents access to emergency funding.

Mr Kear said that yesterday's floods — the second to hit the area in the past six weeks — made such an impact because the water levels rose so quickly and people were caught by surprise. In some parts the water reportedly reached knee-level within minutes.

He described it as a “one in a hundred year event”, but said that despite the severity of the flooding there were no serious injuries because it is a high-flood risk area and “the communities there are well prepared for these situations”.

"This type of flood will only get to that level once in a 100 years,” Mr Kear told The Times. “In saying that, the previous one was just over a decade ago.”

Major flooding in the Coffs Harbour region in 1996 caused $140 million damage and killed one woman. The damage bill for Tuesday’s floods is expected to cost tens of millions of dollars.

Sydney also received heavy rainfall, with the city experiencing its wettest 24 hour period so far this year.

All major beaches from Sydney to the Queensland border — except the popular tourist destination of Bondi — were closed to the general public amid fears over the dangerous waves which battered the eastern coastline.

Surfers took advantage of the massive swell, which reached up to 5m along some beaches. One group took the opportunity to ride a rare wave which had come into Sydney harbour.

About 20 surfers flocked to Nielsen’s Point near Watson’s Bay inside the heads on the southern side of Sydney harbour, to ride the freak wave which occurs in the harbour about once a year.

Surfer Eugene Tan, who runs the Aquabumps website, said that while the three-foot harbour swell was by any means the perfect wave, it was the chance to surf in the harbour while looking at the picturesque Sydney city skyline (and dodging ferries).

“It’s not exactly the biggest or the best, but it’s so unique and happens so rarely it’s really quite special and we just do it for the fun of it,” he said.

More rain is forecast for the next 48 hours and the Bureau of Meteorology has issued severe weather and flash flooding warning for parts of the Sydney area.

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