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Wind gusts of over 100km an hour are set to batter northern New Zealand while the south gets a respite from rain.
The eastern Bay of Plenty is in for a dose of heavy rain tonight after downpours caused flooding, closing parts of State Highway 1 around Oamaru in the sodden South Island today.
Brief heavy showers were expected in the Bay of Plenty overnight, with the most rain set to fall east of Opotiki, the MetService said.
Rain has caused slips on road in the region and one Whakatane house was evacuated today. The rain is expected to ease tomorrow morning and is expected to remain below warning thresholds.
MetService has issued a weather watch for Northland, Auckland, the Waikato, Waitomo and Bay of Plenty areas this afternoon.
The heaviest rain falls are expected to hit the hills around Opotiki but should stay under 100mls, the MetService said.
Wind gusts of up to 110km an hour are forecast for this evening while more rain and gale force winds are heading for the Banks Peninsula, Westland and Fiordland.
Snow warning for South Island
The MetService is predicting a further 80mm to 120mm of rain in the Canterbury foothills and on the hills and ranges of North Otago and Dunedin until tomorrow afternoon.
In addition to the heavy rain, gales were likely about eastern coasts from the Kaikoura Coast to Otago and also in parts of Westland and Fiordland, some gusts were expected to reach up to 130km/h - potentially strong enough to damage trees, powerlines and make driving dangerous.
Heavy rain over Marlborough and the Kaikoura Coast was expected to ease this evening, although another 40mm to 60mm was possible in some places and the wind warning for coastal Wairarapa is lifted.
South Islanders were also warned to brace for snow this week.
Conditions would become progressively colder tonight, with snow showers likely to lower to near 300 metres in eastern parts of Southland and Otago, said MetService forecaster Chris Noble.
"By Thursday morning, snow showers may fall to around 200 or 300 metres in parts of Canterbury and eastern Otago, with some heavy snow possible in these regions.
"This cold outbreak will be the first of the year to deliver snow to low levels in the South Island."
Mr Noble advised farmers that the combination of cold temperatures, snow and strong winds may be stressful to stock.
Travellers should keep up to date with forecasts and check road conditions before travelling tomorrow night or on Thursday.
Lower North Island feeling foul
Rain has spread through the South Island and lower North Island with surface flooding in Canterbury and Wellington.
Wellington is feeling the force of the foul weather with several roads flooded after heavy rains.
Police have said State Highways in Tawa, Churton Park and Petone have been flooded and they are asking drivers to reduce their speed and be patient.
Canterbury's regional emergency management officer Andrew Howe said Civil Defence is winding down and has switched to a "monitoring mode".
He said staff will be on-call overnight and it appears to be the "tail end" of the rain coming down now.
"It is all very quiet," Mr Howe said.
In north Otago's Oamaru, a northern detour for the closed State Highway One had become water-logged, cutting off all northern traffic for the second time today, said the area's civil defence spokesman Richard Mabon.
The fire service's southern communications said callouts to flooding in Oamaru had quietened down.
But it had been called out to a road accident on a wet SH 1 at 1.30pm in Waipapa Bay, near Kaikoura, that had required a helicopter callout.
The service had also attended a flooded address in Oxford, near Christchurch, it said.
St John and Environment Canterbury have yet to confirm or provide further details on either event.
On the positive side, Environment Canterbury spokeswoman Sara McBride said Metservice had advised that the region would get less rain through tomorrow than previously forecast.
But Otago's Mr Mabon said after Wednesday the region was expected to be hit by cold southerlies bringing snow as low as 250m, potentially hurting farm stock.
Bay of Plenty hammered by heavy rains
In the Bay of Plenty, officials in sodden Whakatane are just beginning to assess the clean up job ahead. Some localised areas received in excess of 200 millimetres of intense rain overnight.
Torrential rains flooded some homes overnight, sparking evacuations and more than 20 callouts to emergency services.
One Whakatane family was forced to evacuate and others are thought to have left their homes voluntarily.
"We were on standby to evacuate a whole lot of people. I don't know the extent of the flooding at this stage, but the fire service were very busy pumping out homes both here, in Whakatane, and in Ohope," said Whakatane District Council chief executive Diane Turner.
"We've had lots of streets flooded, slips, there's been some sewage infiltration in a few areas just simply because of the sheer volume of water that came down."
Once in a generation deluge
A weather analyst called it a once in a generation deluge after almost 50mm of rain fell in one hour to midnight.
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.
Wet weather no 'weather 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 Mr 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."