Heavy rain is due to fall on the top of the North Island today - with heavy downfalls forecast from Northland to north of Gisborne, and a double warning for Auckland on Christmas Day.
The MetService has issued a heavy rain watch that covers Northland, Auckland, Coromandel, Waikato, Bay of Plenty and Gisborne.
A low to the north west of the country is causing all the rain, that will continue over the next few days, forecaster James Millward said.
It is slowly moving south, sending fronts down the country.
Mr Millward said rain was going to fall across all of the North Island, and the top of the South Island with some areas likely to experience thunderstorms that may deliver downpours.
"It's going to be a wet couple of days, especially for the northern part of the North Island, which is expected to cop the brunt of the heaviest stuff. But the whole of the North Island will experience this event with some of it expected to spread into the Wairarapa tomorrow."
Aucklanders are warned to prepare for summer storms, including rain on Christmas Day, where downpours are set to coincide with king tides.
Summer storms can change quickly and people should keep a close eye on weather reports, Auckland Emergency Management said.
Driving could become hazardous on coastal roads with king tides around 10am and 11pm on Christmas Day, the emergency management office's acting director Sarah Sinclair said.
Routes such as Esmonde Road and Tamaki Drive may close depending on water levels.
"We're waiting to see how the pressure system develops," she said.
"But we are saying to Aucklanders that if your journey involves any of the areas that [might] be closed for the really high king tides, think about that in your journey planning."
A lot of rain is forecast over parts of the North Island over the next 48 hours. Here's a look at our high resolution rainfall forecast through to midnight #Christmas Some areas may see 100-150 mm+— NIWA Weather (@NiwaWeather) December 22, 2018
-Poor driving conditions
Aucklanders can also take other steps to prevent flooding, such as cleaning drains and gutters and maintaining tree branches, she said.
"If the wind picks up, in advance it might be good to secure any items in the garden that might be affected."
The weather should be better in the South Island on Christmas Day, Mr Millward said.
He said Queenstown and Wanaka were looking like the places to be on Christmas Day with fine, sunny weather predicted for Central Otago. Temperatures may reach the early 30s.
Mr Millward said it was too soon to say what the weather will be like on New Years' Eve, but it would be better than Christmas time in the North Island.