Northland and Auckland are being warned to expect severe thunderstorms and heavy rainfall.
MetService says from this afternoon western parts of Northland and northwestern Auckland could experience up to 40mm of rain in one hour.
It could cause surface or flash flooding, especially for low-lying areas, or lead to slips.
A severe thunderstorm watch is in place between 2pm and 11pm tonight for northwestern parts of Auckland and Northland, spanning between Helensville and Ahipara.
No damaging winds are forecast.
Metservice meteorologist Lewis Ferris said residents in the area need to be monitoring the situation.
"There are going to be places that see more intense weather and some very little," he said.
"The risk is there, that's why we've put out a watch, but it's also understanding not everyone will see the most extreme weather."
There is a bout of unsettled weather forecast for this week, Ferris told the Herald.
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.
He said warm and humid weather this week wouldn't last into the weekend, as cold weather barrels northward.