Saturday afternoon activities could be interrupted by thunderstorms in the North Island as well as the South.
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MetService warned that some of these thunderstorms could become severe, particularly in southern and eastern parts of Taupō, Taihape, inland Whanganui, Hawke's Bay, Manawatu, Tararua District and the Wairarapa this afternoon and evening.
Torrential rain and even hail is possible, spokesman Allister Gorman said.
He added that driving conditions could become hazardous and flash floods and quickly rising waterways are possible.
Thunderstorms with heavy downpours are also forecast for the middle of the country on Saturday, with parts of Nelson and Buller likely to see the worst.
A low over the Tasman Sea is expected to gradually weaken over the day, and an associated front is forecast to bring periods of heavy rain to northwest parts of the South Island.
MetService has issued a heavy rain warning for Nelson west of Motueka, and warned people in Buller that rainfall amounts may also reach warning levels.
The rest of the South Island is expected to have a mostly fine day with some isolated showers in places.
Cricket fans in Dunedin for the Black Caps' one-day match against Pakistan will see a mostly fine day with a chance of showers in the evening – a high of 20 degrees Celsius with a fresh north-eastern wind could make for a good day bowling downwind.
Christchurch has a cloudy start to the morning with a few showers, but then sunny spells should increase to deliver a high of 22C by the afternoon.
Wellington is forecast to reach 24C with a chance of a shower in the northern suburbs from the afternoon.
A high of 26C should bring in a mostly fine day for both Auckland and Hamilton, with a chance of showers in the afternoon and evening.
It's likely to be cloudy in Tauranga, with showers in the morning and later in the evening – a high of 24C is expected.
Looking ahead to Sunday in the North Island, it will be partly cloudy with isolated showers from Northland to Bay of Plenty, and afternoon showers inland further south, some heavy.
In the South Island, it'll be fine in the south and east, with a few showers in the west. Northerly gales are expected to develop about coastal Fiordland.
It's not often Southland can boast about temperatures soaring above 30C, but by Sunday people in Invercargill should be ready to crow.
Invercargill is forecast to reach 30C on Sunday and Monday for only the fifth time in over 20 years, as a Foehn heating system warms the southern region.