Scattered thunderstorms are expected about parts of the North Island on Thursday, with localised heavy rain and hail.
MetService issued heavy rain warnings for Gisborne, Wairoa and Hastings; this would last until noon Saturday in Wairoa, and to 4am Friday in Gisborne and Hastings.
It also issued a severe thunderstorm watch for the Coromandel Peninsula, Waikato, Waitomo, Bay of Plenty, 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 heavy rain 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.
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."
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.
Heavy rain warnings on East Coast
In Gisborne/Tai Rāwhiti, heavy east to northeast rain was expected to set in from around 6am, becoming south to southeast late on Boxing Day.
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.
