3.50pm update:  At 03.37 pm, MetService weather radar detected severe thunderstorms near the Mamaku Plateau. 

"These severe thunderstorms are moving towards the northwest, and are expected to lie near Tirau, Putaruru. the Mamaku Plateau, Te Poi and Okoroire at 4.07pm and near Matamata, Tirau, Putaruru, Te Poi, Okoroire and Hinuera at 4.37pm," says a MetService spokesperson.

"These thunderstorms are expected to be accompanied by very heavy rain, hail and frequent lightning."

Very heavy rain can cause surface and/or flash flooding about streams, gullies and urban areas, and make driving conditions extremely hazardous.

Hail can cause damage to crops, orchards and vines and may make driving conditions hazardous.

Frequent lightning is dangerous to people and livestock, especially those exposed outdoors or on the water, and can cause power surges and damage to electrical equipment.

A Severe Thunderstorm Watch remains in force for Auckland, Coromandel Peninsula, Waikato, Waitomo, Bay of Plenty and Rotorua.

The National Emergency Management Agency advises that as storms approach you should:
- Take shelter, preferably indoors away from windows;
- Avoid sheltering under trees, if outside;
- Get back to land, if outdoors on the water;
- Move cars under cover or away from trees;
- Secure any loose objects around your property;
- Check that drains and gutters are clear;
- Be ready to slow down or stop, if driving.
During and after the storm, you should also:
- Beware of fallen trees and power lines;
- Avoid streams and drains as you may be swept away in flash flooding.

Earlier, 1pm:  MetService has issued a Severe Thunderstorm Watch for Bay of Plenty, Coromandel Peninsula, Auckland, Waikato, Waitomo, and Rotorua.

This watch is valid for seven hours from 1pm - 8pm today, Sunday, October 8.

Thunderstorms are expected to develop over the north of the North Island early Sunday afternoon.

These thunderstorms are likely to be accompanied by heavy rain with intensities of 15 to 25 mm/h, and hail 10 to 20 mm in diameter.

Additionally, about Waitomo north of Te Kuiti, Waikato, Bay of Plenty, Coromandel Peninsula, and in the south and southeast of Auckland - from about Howick to Papakura and south of the Manukau Harbour, there is a moderate risk of thunderstorms becoming  severe this afternoon and early evening.

These severe thunderstorms could bring localised downpours of 25 to 40 mm/h, and the also the possibility of localised tornadoes.

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.

Tornadoes may cause some structural damage, including damage to trees and power lines, and make driving hazardous. If any tornadoes occur, they will only affect very localised areas.

Note: 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.

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