Exceptionally heavy rain and wind gusts up to 130km/h are set to batter parts of the South Island today as a Tasman tempest makes landfall.
MetService is forecasting the largest amounts of rain for parts of the island with Fiordland and West Coast set to take on up to 700mm between Sunday and Tuesday.
A cold front was also expected to bring northwesterly winds, rising to severe gale in exposed parts of Fiordland, Southland, Otago west of Alexandra, and the Canterbury High Country.
From 9am today until 11pm tomorrow, some 500mm to 700mm was forecast from Westland to Otira about the ranges, and 200mm to 300mm near the coast.
Rain intensity could hit 20mm to 30mm per hour about the ranges today and 30mm to 40mm per hour tomorrow.
Up to 600mm was forecast about the headwaters of the Canterbury Lakes and Rivers south of Arthur's Pass for the same period, and up to 500mm about the headwaters of the Otago lakes and rivers to 5pm tomorrow.
Parts of Fiordland could see up to 400mm by 1pm tomorrow.
The Canterbury high country could also see wind gusts to 130km/h until 11pm tomorrow.
The wild weather was being driven by warm, moist northwesterlies ahead of an active front over the Tasman Sea.
The cold front would slowly sweep over the South Island before stalling over the North Island about mid-week.
NIWA said the “recipe” for the prolific rainfall was a combination of an atmospheric river extending from Australian cyclones, extra energy from the Tasman Sea marine heatwave, and a strong low pressure system siphoning moisture toward New Zealand.
MetService meteorologist Stephen Glassey said once the system arrived in the North Island from Wednesday, it would have severely weakened but still would bring some rain and showers to places.
However, the time had not yet come to pack away the shorts and T-shirts, with more warm weather forecast.
Temperatures would remain consistently in the mid to high 20s for the upper North Island, and in the low 20s for much of the rest of the country.
Eastern parts of the country were in for some warm and dry weather today. In the South Island, the east coast was set to cook, thanks to the foehn effect driving warm and dry air from the Southern Alps. Christchurch was forecast to hit 29degC.
Meanwhile, WeatherWatch NZ was predicting the tail of ex-cyclone Trevor could drift east and join forces with a cold front in the Tasman Sea on Friday, sparking a new rain band and possibly a new low pressure system in the New Zealand area.
