A complex low-pressure system forming over the Tasman Sea is set to unleash a “river of moisture” over parts of the country.
The National Institute of Water and Atmospheric Research (Niwa) said the system, combined with “abundant moisture” and New Zealand’s mountainous terrain, is expected to produce significant rainfall, particularly in western parts of the South Island.
“Meanwhile, colder air near the surface may be just enough to support snowfall across inland Otago and parts of the Canterbury High Country,” Niwa said.
Up to half a metre of rain is expected to fall in two days on the West Coast while heavy snow is forecast to blanket parts of Canterbury and central Otago.
The Westland area is under an orange heavy rain warning for 48 hours from noon tomorrow, with 400-500mm of rain, surface flooding, slips and dangerous driving conditions expected.
A heavy snow watch is in place for South Canterbury high country and North Otago for almost two days, which could see snow heavy enough to cause travel disruption and damage to trees and powerlines.
A complex area of low pressure developing in the Tasman Sea is set to send a river of moisture across NZ over the coming days.
— NIWA Weather (@NiwaWeather) August 25, 2026
The combination of abundant moisture, low pressure, & mountainous terrain is expected to produce significant rainfall in the west of the South Island pic.twitter.com/4wau7PXt1M
A five-hour snowfall warning has also been announced for Crown Range Rd from noon tomorrow.
MetService warned to “prepare for snow, cold temperatures and possible power outages”.
“If you must travel, drive cautiously and ensure you have snow chains, sleeping bags, warm clothing and emergency items.”
NZ Transport Agency Waka Kotahi (NZTA) warned motorists of potential disruption from heavy snowfall and rain later this week.
MetService meteorologist Alwyn Bakker told the Herald that much of the North Island was in for a pleasant day, particularly in eastern and northern areas.
“In the west, there might be the odd shower but it’s mostly just a bit cloudy,” he said.
“Down south, it’s a bit of a different story. There is going to be rain for the West Coast and maybe a little bit of rain through Southland, Otago and high cloud for the East Coast.”
Ashburton and Timaru were forecast to reach 21C today, about 8C above average for this time of year, while Christchurch tops out at 20C, around 7C warmer than usual.
But the warmth will be short-lived, with a low-pressure system moving over the South Island from tomorrow.
Bakker said the storm would be “quite significant”, with a “solid potential for disruption”.
“Don’t panic, pay attention,” he said.
Unlike a straightforward westerly flow, the rotating low could draw colder air from further south as it crossed the country, helping set the stage for heavy rain and snow.
Bakker said heavy rain would hammer the West Coast, while warning-level amounts of snow were possible from Central Otago and the Queenstown Lakes District north to the Mackenzie Basin.
“Instead of being a nice light dusting, it’ll ... make it difficult to go places, [and] might be a threat to livestock,” Bakker said.
Snow is forecast to lower to about 700m late tomorrow and potentially 400m on Thursday.
NZTA has separately warned that private forecasting suggests more than 30cm of snow could affect SH8 and SH80, while up to 20cm could fall on SH73 above 700m.
“Accumulations at this level are likely to impede travel and NZTA may close sections of highway or introduce restrictions to keep people safe and avoid vehicles becoming trapped,” it said in a statement.
A 36-hour heavy rain watch is also in place for the ranges of Westland from noon tomorrow, which MetService expects to upgrade to a warning today.
Bakker said uncertainty remained around exactly where the low would track and which areas would receive the greatest rainfall.
While such a storm was “not super unusual” approaching spring, he said its impacts could nevertheless be substantial.