Scattered thunderstorms are expected about parts of the North Island on Thursday morning, with localised heavy rain and hail.
MetService has issued a severe thunderstorm watch for the Coromandel Peninsula, Waikato, Waitomo, Bay of Plenty, and Rotorua.
Dunedin can expect the low cloud to lift this morning then the odd shower. The southwesterly wind will become strong about the peninsula this afternoon.
And in Christchurch, it will be mostly cloudy with periods of rain, possibly heavy, and fresh south-westerlies.
However, MetService said the east coast of the North Island may see downpours of 25 to 40mm/h, or possibly even more.
"Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.
Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain."
MetService said a broad area of low pressure was bringing unsettled weather to many parts of New Zealand on Boxing Day and Friday, while a low was deepening east of the North Island.
Hawke's Bay officials have been preparing for possible flooding, with the regional council calling back staff from leave after MetService issued heavy rain warnings.
"Our team has been called back from leave and includes staff on the ground in Wairoa, closely monitoring river levels and looking to potentially open minor river mouths in the area should the need arise," Hawke's Bay Regional Council said in a statement on Wednesday.
"Currently the Wairoa River mouth is performing well and we will be closely monitoring the mouth during the period of high swell.
"From Esk to Māhia, we expect rivers to rise and remain within their channels, although we urge the community to be mindful that levels may rise quickly in heavy rain."
MetService has issued an orange heavy rain warning for Gisborne/Tai Rāwhiti, the Hastings District north of Napier, and the Wairoa District on Thursday.
A strong wind watch has also been issued for the coastal areas of Hawke's Bay, Gisborne/Tai Rāwhiti, Taranaki and western parts of Whanganui, and Kapiti Coast, Wellington and coastal Marlborough including the Sounds.
In Gisborne/Tai Rāwhiti, heavy east to northeast rain set in from around 6am, and will become south to southeast late on Boxing Day.
Thunderstorms are also possible.
While in the Wairoa District, 200 to 250 mm of rain was expected from 10am on Thursday until noon on Saturday.
80 to 100 mm of rain was also expected from 10am on Thursday in the Hastings District north of Napier.
MetService warned surface flooding, slips and difficult driving conditions were possible.
However, it said there was only a minimal chance of upgrading to a red warning.
"We're closely monitoring the situation and will share updates if anything changes," the Hawke's Bay Regional Council said.
In Taranaki and western parts of Whanganui, southerly winds may approach severe gale in exposed places, with a moderate chance of upgrading to a warning until 4pm on Friday.
A similar watch is in place for coastal areas of Hawke's Bay and Gisborne/Tai Rāwhiti from around 8am on Friday, with a moderate chance of the strong wind watch being upgraded to a warning.
As well as for Kapiti Coast, Wellington and coastal Marlborough including the Sounds between 1am and 4pm on Friday.
