New Zealand can expect a lot of wet weather for the next few weeks, but conditions turn much drier in October.
Government forecaster Niwa is predicting above normal rainfall for almost the entire country next week, with only the east coasts of the North and South Island receiving below normal rainfall.
Forecast rainfall anomalies (i.e. rainfall difference from normal) over the next 35 days are shown here in five weekly chunks
— NIWA Weather (@NiwaWeather) September 15, 2025
The next 2-3 weeks are expected to remain wet (🟢) in the western South Island due to an active Southern Ocean
Drier conditions are favoured in October pic.twitter.com/lyMEZxq9W7
Much of the South Island will take a battering during this period, with the western parts of the island being the wettest part of the country “probably for the rest of the month, maybe even to the first few days of October”, said Niwa’s principal scientist, Chris Brandolino.
“There will be probably a wet period for the North Island, maybe the 21st to the 28th [of September], so that week could be an unsettled and wet week.”
The bad weather has started this week as the South Island was predicted to have above normal rainfall all week.
MetService has put orange heavy rain warnings in place for much of the South Island’s West Coast, while the North Island escapes most of the inclement weather.
Severe Weather Update 🟠🟡 🌧 Orange Heavy Rain Warnings: - The ranges of Westland and Fiordland north of Doubtful Sound. -...
Posted by MetService New Zealand on Monday 15 September 2025
Niwa’s principal scientist said that the Canterbury High Country, higher elevations in Otago and Wellington were due for strong winds this week.
⚠ Severe Weather Update
— MetService (@MetService) September 15, 2025
The next couple of days bring brighter skies, then winds are forecast to strengthen on Wednesday as wet weather spreads north 🌧️💨
Initial Wind and Rain Watches have been issued for Canterbury and the Southern Alps.
🔗https://t.co/qHyE5zzql5 pic.twitter.com/K3m429SnCG
Brandolino told the Herald that the wet weather, and the strong west wind that come with it, are a result of two systems of air pressure – what he calls “cogs” – working together.
“Over the Southern Ocean, we are expecting one cog, and that cog is low pressure over the Southern Ocean, and that’ll kind of come and go.
“Now, to the north of the country, over say the Coral Sea and between Northern New Zealand and New Caledonia, so kind of to our north, we’re expecting a high pressure cog. And those two cogs are going to work together to give us that west wind.”
These two systems will work together for the rest of September, according to Brandolino, and create blustery conditions for much of the country.
“When you have high pressure and low pressure kind of close together, the change in the air pressure ... can be quite dramatic, and that’s kind of like putting your thumb over part of the garden hose – it makes that fluid, this time the air, really accelerate.”
The east coasts of both islands may have the best weather during this period and are likely to be “spectators” of the heavy rain.
“Not to say they won’t get any rain,” said Brandolino, “but compared to what falls over the western South Island for the rest of September, and what falls over the North Island that last week-ish of September, I think the eastern part of both islands, they’re going to be largely spectators.”
Things might start to look up in October, with much drier conditions predicted due to the low pressure system in the Southern Ocean disappearing.
“I don’t think it means it’ll be dry for 28 straight days or anything like that. Of course, there’ll be bouts when it does rain or there is a bit of a change in the pattern, but that will be the exception and not the rule.
“So, almost a reversal of what we’re experiencing or we’re about to experience and have experienced is probably a good way of looking at it for a good chunk of October.
“Some days it’ll feel almost like summer, and then it goes back to winter. And that’s, that’s kind of, that’s kind of quintessential with spring.
“It’s quite variable and quite moody. I call it the teenager of seasons because lots, lots of ups and downs”.