A sub-tropical storm may be on a collision course with the North Island this week.
Niwa has issued a heads-up to holidaymakers saying models show the storm, fuelled by an atmospheric river from the tropics, is set to come near the North Island this Friday.
The storm, packed with strong northeasterly winds and heavy rain, could sweep over the North Island and parts of the South Island this Thursday and Saturday.
Holiday hotspots Coromandel Peninsula, Bay of Plenty and Gisborne could be in line for some of the wettest weather.
Niwa said the coming storm may be an "impactful" system.
Niwa tweeted there was the potential for an "atmospheric river of moisture" extending from Australia and New Caledonia down to New Zealand which threatened to bring significant rainfall this week.
The week is already off to a gloomy start for much of the country as a frontal system sweeps up the country bringing thunderstorms and rain to both islands.
But this is set to get worse if the tropical-fuelled low sweeps down the country. Some models show it stalling off the North Island's west coast, however Niwa says the warmer-than-average sea surface temperatures could mean the storm is stronger than otherwise expected.
In its severe weather outlook MetService said a slow-moving low and associated front was set to move slowly over the upper North Island on Friday.
Strong northeasterly winds would precede the weather system, which was expected to bring rain to much of northern and central New Zealand on Friday. At times it would be heavy.
Ahead of the storm some of the biggest tides of the year are set to swamp coastlines from tomorrow until the weekend.
Niwa warns with the potential for onshore winds and a significant low pressure system there may be coastal flooding to low-lying areas.
Today the western coastline of the country is being swamped by powerful waves that have been fuelled by a storm in the Southern Ocean. The waves may be so dangerous people are being warned to keep off rocks and ledges near beaches or attempt bar crossings in boats.