MetService forecasters are watching a complex low pressure system over the country, which has been delivering significant rainfall to the upper North Island.

The low is expected to move southeast and away from the North Island by Wednesday, as a welcome ridge of high pressure builds over New Zealand from the Tasman Sea.

The warm, humid air over the country brings the risk of thunderstorms accompanied by localised heavy rain to many inland North Island areas this afternoon and evening, while today the risk becomes confined to Bay of Plenty, Taupo and the Gisborne Ranges.

High humidity over eastern parts of the country will bring extensive low cloud and patchy drizzle tonight and into Tuesday, with sea fog likely in onshore flows around the coast from Marlborough and Wellington through to Wairarapa and southern Hawke's Bay.

"The good news is that a ridge of high pressure makes its way onto the South Island from the Tasman Sea on Wednesday, then moves north over the North Island on Thursday,” says MetService Meteorologist Andy Best.

'Temperatures are expected to stay relatively mild over the country this week for this time of year, with Westland seeing highs in the low-20s while the Canterbury Plains and much of the North Island experience highs in the mid-20s.”

Looking further ahead, an area of low pressure approaches the west coast of the South Island on Friday, although at present there is uncertainty as to its position.

Forecasts for the weekend will be updated with the development of this system as the week progresses.　

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