MetService has issued a "severe thunderstorm watch" covering Northland, Auckland, Great Barrier Island, the Coromandel Peninsula, Waikato and Bay of Plenty.

The warning covers the period from 7pm Friday, through to 8am on Saturday.

In its warning, MetService said: "An unstable airmass with embedded troughs is expected to affect the upper North Island this evening, overnight and into Saturday morning. In much of Northland, Auckland, Coromandel, the north of Waikato and the far west of the Bay of Plenty.

"Thunderstorms are likely and there is a risk that some of these could be accompanied by downpours with rainfall intensities of 25 to 35mm per hour and tornadoes with strong wind gusts.


"Wind gusts of this strength can cause some structural damage, including trees and power lines, and may make driving hazardous. If any tornadoes occur, they will only affect very localised areas."

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Meanwhile, rain this week has topped up Auckland's critically low reservoir dam levels.

Watercare said the region's dams were 47.1 per cent full, compared to 44.8 per cent on Wednesday. The normal average level for this time of year is 78.53 per cent.