Northland and Auckland are being warned to expect severe thunderstorms and heavy rainfall.
There are dangers of flash flooding with today's expected thunderstorms. Photo: Freddie Marriage/ Unsplash
MetService says from this afternoon western parts of Northland and northwestern Auckland could experience up to 40 millilitres of rain in one hour.
It could cause surface or flash flooding, especially for low-lying areas, or lead to slips.
The warning is active until 11pm tonight.
The severe weather marks the first day of autumn and the rest of the week is expected to turn cold for many across the country.
NIWA meteorologist Chris Brandolino said as the season officially changed the weather would shift rapidly as we open the doors to sub-antarctic weather.
He said warm and humid weather this week wouldn't last into the weekend, as cold weather barrels northward.
"It will certainly be a shocker because the days preceding that will still be on the warm, humid side, particularly north. In fact over the next couple of days during mid-week there will be episodes of showers and thunderstorms and humidity."
He said daytime highs for South Island cities like Dunedin, Queenstown or Invercargill would struggle to reach double digits at the weekend.
Strong south-west winds at the end of the week and throughout the weekend for the South Island and lower parts of the North Island were likely to make the cold snap even more intense.