A sub-tropical storm with hurricane-strength winds is taking aim at New Zealand. The weather is expected to pack up today before the storm makes landfall tomorrow.

The storm is on track to hit the top of the country tomorrow before sweeping down the country, affecting all the North Island and the north and east of the South Island.

Heavy rain is expected to drench Northland and Auckland tomorrow before spreading across the island. Potentially destructive winds are expected to barrel across the North Island for days.

Waves up to 7m high are expected to hammer North Island coastlines.

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Coastal areas are also at risk of flooding from the sub-tropical storm, which is bearing striking similarities to ex-Tropical Cyclone Ita that inundated parts of Auckland in 2014.

MetService says heavy rain warnings are likely for Coromandel Peninsula, Bay of Plenty and the western ranges of Gisborne when the storm hits late Thursday.

The country's forecaster is warning the storm could bring severe weather to northern and central New Zealand.

Weatherwatch.co.nz head analyst Philip Duncan said the storm would rapidly deepen and intensify before it made landfall tomorrow.

Campers and trampers were warned to be aware of the deteriorating conditions.

Duncan said models showed the storm was expected to peak early Friday. A wind map put the worst of the winds becoming hurricane force - 120 km/h - for a time at the centre, although such winds would stay mainly out at sea.

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The North Island is in line for more thunderstorms today when daytime heat will lead to large cloud build-ups and isolated torrential downpours with thunderstorms.

Bay of Plenty and Gisborne, western North Island areas from Waitomo to northern Manawatu are at greatest risk along with regions at either end of the South Island.

A severe thunderstorm watch is in place for the central North Island from Taupo to Manawatu including Hawke's Bay and Taranaki.

MetService says any thunderstorms that develop will be slow-moving and may produce localised heavy rain or downpours.

The rain could be so intense people are being warned to expect up to 40mm in one hour in western and central North Island.

The forecaster says this level of drenching could lead to flash flooding and make driving conditions extremely dangerous.

The thunderstorms are expected to die down late evening.

Duncan said the approaching storm was likely to cause very dangerous marine conditions, bringing strong rips and mountainous seas to the western and eastern coastlines of the North Island.

He said those near streams should be prepared to move to higher ground should the low produce flooding rains, which is possible in some spots - especially after such a long dry end to spring and start to summer.

Niwa today said the strong onshore northeasterly winds meant king tides due to hit east Auckland on Thursday night and Friday morning threatened to cause major flooding.

There were similarities between the upcoming sub-tropical storm and the remnants of Ita, which hit in April 2014 and inundated Tamaki Dr.