Scattered thunderstorms are expected to develop over many parts of the northern and central North Island this afternoon and evening, says a MetService spokesperson.

These thunderstorms will be accompanied by heavy rain and have the potential to produce localised downpours in excess of 25mm per hour.

The highest risk of downpours is about Central and Southern Hawkes Bay from about Hastings southwards, and the Tararua District between about 1pm and 8pm today, Sunday December 24.

A Severe Thunderstorm Watch is now in force for these areas.

Rainfall rates of 25-40mm per hour, or possibly more, can be expected with the thunderstorms.

Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.

Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain.

The thunderstorm activity should die away this evening, however further thunderstorms are likely tomorrow - Christmas Day.

A Severe Thunderstorm Watch means conditions are favourable for severe thunderstorms in and close to the watch area.

People in these areas should be on the lookout for threatening weather conditions and monitor for possible Severe Thunderstorm Warnings.

Heavy rain for parts of central New Zealand and Westland

A humid northerly flow spreads onto New Zealand through the weekend, bringing rain to many places in the north and west. Heavy rain is expected about Mount Taranaki/Taranaki Maunga, the Tararua Range and the ranges of the northern South Island and Westland.

People are advised to keep up to date with the latest forecasts in case further areas are added or upgraded.

Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.

Heavy Rainfall Warnings and Watches are in place for these regions:

Area: Mount Taranaki/Taranaki Maunga
Valid: 28 hours from 2am Sunday, December 24 to 6am Monday, December 25
Forecast: Expect 120 to 150 mm of rain near the summit, with lesser amounts at lower levels of the Mountain. Peak intensities of 15 to 25 mm per hour and possible thunderstorms during Sunday afternoon and evening.

Area: The ranges of Westland south of Otira
Valid: 28 hours from 2am Sunday, December 24 to 6am Monday, December 25
Forecast: Expect 150 to 200 mm of rain to accumulate. Intensities generally reaching 10 to 15 mm per hour, but may increase to 20 to 25 mm per hour Sunday afternoon and evening with possible thunderstorms.

For information on preparing for and keeping safe during a storm, see the Civil Defence Get Ready website