According to MetService, there is a risk of thunderstorms spreading southwards over the northern North Island, mainly during the afternoon and evening, but also at night around the Taranaki and Bay of Plenty regions.
These thunderstorms could be significant, with localised heavy rain, hail, strong squally wind gusts and possibly a few small tornadoes.
The organisation attributed the conditions to a big low from the Tasman Sea moving over the country.
MetService forecaster Aidan Pyselman said this kind of unstable atmosphere leads to very heavy showers, thunderstorms, lightning and squally conditions.
“The nature of the set-up means we can’t pinpoint where it’s going to happen, so it’s quite broad-brush over large parts of the country.”
Niwa (National Institute of Water and Atmospheric Research) recorded more than 800,000 lightning strikes in the 24 hours to 1pm on Sunday.
And over the Easter long weekend, Auckland and Tasman were plagued by freak tornadoes that brought down trees and damaged houses.
However, Auckland Emergency Management said there is still a chance of more tornadoes to come.
Group controller Mace Ward said it is hard to pick where the greatest risk is and almost impossible to predict where a tornado might appear.
The organisation’s advice if you see a tornado is to seek shelter immediately. If you’re inside, stay away from doors and windows, and stay downstairs if you’re in a multi-storey building.
For added protection, get under something sturdy, cover your body with a blanket or mattress, and protect your head with your hands.
If you’re outside, lay flat in a gully, ditch or low spot on the ground and protect your head with an object or your arms, and if you’re in a car, pull over and shelter in a low spot.
Pyselman said it’s looking like a fairly unstable week.
He said the low which is affecting the country will gradually cross and weaken away on Wednesday.
“But there are still fairly unsettled-looking conditions, with a lot more shower activity over both islands.”
MetService wants people to continue to check their website for weather updates and call 111 if anyone is in immediate danger.