WELLINGTON (Reuters) - New Zealand began a major clean-up operation on Friday after heavy rain and floodwaters from the tail end of Cyclone Debbie swamped towns in the North Island and caused landslips that blocked roads.

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The town of Edgecumbe in the Bay of Plenty was almost completely underwater after river levels peaked overnight, leaving thousands of evacuated residents homeless for what is likely to be several days.

Elsewhere across the island, emergency services were working to clear blocked roads and restore power to isolated communities. Medical supplies and food were being flown in by helicopter.

Prime Minister Bill English was due to visit Edgecumbe, where rescue workers used tractors and boats to evacuate thousands of people in what meteorologists said was a once-in-500-year event, to inspect the damage later on Friday.

Cyclone Debbie, a Category 4 storm, first slammed into northern Australia more than a week ago. The storm, one level shy of the most powerful Category 5, and associated flooding smashed tourist resorts, brought down power lines and killed six people in Australia.

The storm system, downgraded from cyclone level, then crossed to neighboring New Zealand, where mountainous terrain makes its roads susceptible to landslides and many regions are still recovering from November’s 7.8-magnitude quake. No deaths have so far been reported in New Zealand.

Kaikoura, the coastal holiday town at the epicenter of that quake, was shut off from the rest of the country for the second time in six months as connecting roads were again hit by landslips.

Back in Australia, the disaster zone stretched 1,000 km (600 miles) from Queensland state’s tropical resort islands and Gold Coast tourist strip to the farmlands of New South Wales state.

The deluge took days to flow through tropical river systems and water levels peaked in the city of Rockhampton at lunchtime on Thursday, flooding main streets, shops and homes.

Residents rowed boats along main roads and muddy water covered the airport’s runway. Authorities said the airport will be closed for six days and the water is not expected to recede until the weekend.

Australian insurers have declared the event a catastrophe likely to cost more than one billion dollars, with state officials saying recovery and repairs will take months.

The storm is also having a longer-lasting effect on Australia’s coal exporters with four miners in the region this week declaring force majeure - a clause typically invoked after natural disasters - leaving rivals in the United States to cash in on a surge in prices as Chinese steelmakers scramble for supplies.