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Torrential rain has flooded homes overnight in the Bay of Plenty, sparking evacuations and more than 20 callouts to emergency services.
A weather analyst is calling it a once in a generation deluge after almost 50mm of rain fell in one hour to midnight.
Other parts of the country are due to get more serious weather this week, with a thunderstorm watch issued for Auckland and Taranaki and flooding predicted in South Canterbury.
WeatherWatch analyst Richard Green said Whakatane had been particularly hammered overnight.
"For a one hour period, just before midnight last night, 46mm of rain fell and that doesn't happen too often at all."
Whakatane's War Memorial Hall has been opened as an evacuation centre for people forced to leave their homes because of flooding.
Fire Service staff dealt with 22 callouts to flooded homes and shops in just a couple of hours. The last callout was at 1am.
Emergency services continue to clear slips on motorways, including State Highway 3 in the Manawatu Gorge. Road authorities have issued travel warnings and are working to help motorists.
The MetService has issued a thunderstorm watch for Auckland and Taranaki.
"We had our entree [Sunday night and yesterday morning]. Our main course is still to come," said MetService ambassador Bob McDavitt.
"The next step is the westerlies."
Westerly winds would arrive tonight bringing big squally showers and heavy swell to the north's West Coast beaches.
Mr McDavitt warned surfers to avoid the West Coast tomorrow and Thursday.
"You won't get a whole day of heavy rain but you will get a bucket of rain in a short time," he said.
The strongest winds reached 94km at Cape Reinga at 6pm on Sunday.
But the serious damage was expected in the South Island, Mr McDavitt said.
"Everybody else is just [receiving] superficial damage."
A front carrying heavy rain would stall over the Canterbury Plains for the next two days, he said.
Christchurch and the surrounding areas were at risk of floods after Christchurch got 42mm of rain in the 24 hours leading up to 5pm yesterday, with two more days of heavy rain ahead.
Timaru would get more than a month's worth of rain in a single day, said Mr McDavitt.
The Otago Regional Council is advising people to stay alert as widespread flooding is predicted.
The Kakanui River, south of Oamaru, is high, with the catchment receiving 100 millimetres of rain overnight. The council's duty flood manager Matt Dale said flows were picking up by the minute. He said flooding was expected to happen over the next few hours and Civil Defence would be on alert.
Big impact but not a 'bomb'
North Islanders may have felt like they were in the middle of a weather explosion when they were hit by heavy rain on Sunday night.
But the drop in low pressure was less than half what it would take to qualify as a "weather bomb".
According to MetService weather ambassador Bob McDavitt, bad weather is officially called a meteorological weather bomb when a low pressure system deepens by 24 hectoPascals or millibars (hPa) during a 24-hour period.
He said the last true bomb in New Zealand was in late July 2008. That cost an estimated $42 million in insurance payouts.
He said the term bomb had its early beginnings when Swedish meteorologist Tor Bergeron coined the measurement of 24 hPa in 24 hours as a unit for gauging quickly deepening lows in the centre of storms in the North Sea.
These days meteorologists say a pressure change of 24 hPa equals 1 Bergeron, or 1 "b" - otherwise known as a weather bomb, said Mr McDavitt.
He noted people had started using the term to refer to all kinds of bad weather events.
The central pressure of a low was only one measure of how much damage a storm would cause, he said.
The low this week would deepen further by tomorrow.
"It is definitely a major event."
