This would see localised heavy downpours of 25 to 40mm per hour, as well as a chance of large accumulations of hail.
MetService said the deluge could lead to flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.
It warned driving conditions would become hazardous as motorists navigated surface flooding and poor visibility in heavy rain.
The hailstorms were likely to cause significant damage to crops, orchards, vines, glasshouses and vehicles, as well as make driving conditions hazardous.
Heavy rain had saturated the Bay of Plenty overnight with more than 100mm falling in some places.
While much of the island was expected to be peppered with spring showers across the day the focus was on the changeable and turbulent conditions overhead that would also affect the top of the South Island.
This morning Waka Kotahi NZTA said the bad weather was causing havoc on state highways in eastern Bay of Plenty.
The roading agency asked motorists to take care on the region's roads after heavy rain closed a section of SH30 between Western Drain Rd and State Highway 2, as well as affecting parts of SH2 Waimana Gorge which was now down to one lane due to slips and fallen trees.
A detour was in place for the SH30 closure with motorists asked to drive carefully and allow additional time for the alternate route.
The stormy weather comes as Niwa releases its quarterly climate forecast as the country heads into summer.
It includes the prospect of temperatures likely to be above average across the country, with particularly warm weather from the second week of November, and the potential for another marine heatwave.
Niwa said the South Island and west of the North Island were likely to be in for more dry spells while occasional sub-tropical low pressure systems were expected to bring heavy rainfall and possible flooding, particularly to the north and east of the North Island.
The forecaster warned the tropical cyclone season that runs from November through to April showed the country was at an elevated risk of being directly affected.
MetService said after today's stormy weather the country was in for a mainly settled Saturday before the next front swept overhead bringing a period of wet weather followed by some cooler temperatures.
Forecasters said parts of the country could expect to see up to a 10C drop in maximum temperatures.
