Forecasters are urging Kiwis to brace themselves for a weather bomb, with the MetService issuing severe weather warnings for both the North and South Islands.

Heavy rain and gales are expected to lash both islands from tonight, which the MetService warned would be one of the "bigger weather events" for some time.

Streams and rivers would rise quickly, and there could be surface flooding in low-lying areas.

The weather was likely to make driving dangerous, causing slips in hilly areas, and damaging trees and powerlines.

Winds of up to 90kmh are expected to lash the North Island from Auckland to Kapiti, with severe gusts expected in Taranaki and Manawatu.

Storms are forecast for Northland, north Auckland and Great Barrier Island this evening, with up to 90mm of rain predicted in some hill areas.

Coromandel, Bay of Plenty and Gisborne were expected to be buffeted tonight and tomorrow morning, with the storm easing in the afternoon or evening.

About 100mm to 150mm of rain was expected near the ranges, with heaviest falls north of Gisborne.

Areas of the South Island are also expected to be hit, including Canterbury, Marlborough, Westland, Buller and Nelson.

Nelson Tasman Civil Defence tonight issued a warning for residents in the Tapawera area, and had contacted residents affected by a flood in the area last weekend.

Environment Canterbury would use excavators to open the river mouths of Saltwater Creek south of Timaru and Washdyke Creek north of Timaru tomorrow morning ahead of expected flooding.

The rain was expected to cause severe flooding in North Otago catchments, possibly extending to the Dunedin and Taieri catchments, the Otago Regional Council advised.

The biggest rainfalls would be in Nelson, Marlborough, Canterbury and Otago tomorrow and Tuesday, with up to 200mm expected in places.

People are being warned to stay up-to-date with weather forecasts over the next 24 hours.

Harry O'Rourke, group controller for the Auckland region civil defence emergency management group, advised people to be prepared.

"It is a good time to check your emergency survival kit and update your household emergency plans. Make sure your battery-powered torches and radios are working and check the expiry dates on all food in your emergency survival kit," Mr O'Rourke said.

An emergency survival kit should contain food and water for three days or more, a battery-powered torch and radio, first aid kit/medication and supplies for pets.

People living in low-lying areas prone to flooding were advised to have a getaway kit ready in case they needed to move to higher ground.

The Civil Defence emergency management office will continue to monitor the weather situation.