MetService has issued a severe thunderstorm watch for Bay of Plenty and Rotorua.

"An active front with a band of heavy rain and squally thunderstorms is expected to move southeastwards over the northern and central North Island overnight," says a MetService spokesperson.

The watch is valid for seven hours from 10pm Sunday April 9 to 5am Monday April 10.

"For Bay of Plenty and Rotorua, there is a risk that some of the thunderstorms could be severe between about 10pm tonight and 5am Monday morning, with localised downpours of 25-40mm per hour.'

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.

"These thunderstorms may also bring squally wind gusts of 90-100km oer hour, hail, and frequent lightning. There is also a low risk of one or two small localised tornadoes, mainly near the coast.

The thunderstorm activity should ease from the west overnight.

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.

MetService has also issued a severe weather outlook for the days ahead.

"A large trough lies over the country for most of the outlook period," says a MetService spokesperson.

"This will bring unsettled weather to many places. A weak high may move onto the South Island during Friday."

MetService says there is moderate confidence that warning amounts of rain will fall in the west of the South Island from Tasman to southern Westland during Tuesday. Further rain is possible in this area on Wednesday, but there is only low confidence that a rainfall warning will be needed.

On Tuesday, there is low confidence that severe gale northwesterlies will affect Wellington and the Marlborough Sounds.

For Mount Taranaki and the Tararua Range there is low confidence that a rainfall warning will be needed on Tuesday. There is also low confidence that a rainfall warning will be needed for eastern Bay of Plenty during Tuesday and Wednesday.

During Thursday and Friday, there is low confidence that warning amounts of rain will fall in eastern parts of the South Island from Canterbury to Clutha, with the heaviest rain more likely in the south of that area on Thursday and in the north on Friday.