Many parts of the country can expect to get a soaking of heavy rain on Christmas Eve and Christmas Day.
MetService said a humid northerly flow is spreading onto New Zealand through the weekend, bringing rain to many places in the north and west.
This has seen MetService issue heavy rain warnings and watches for parts of the North and South Islands.
Auckland is under a severe weather watch from 1pm until 8pm on Sunday with potential for localised downpours of 25-40mm/h. The risk should die away after 8pm but there is a further risk of thunderstorms and downpours over Auckland again on Christmas Day.
MetService said downpours could occur with or without thunderstorms and are more likely about northern and eastern parts of the Auckland region.
"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."
A severe thunderstorm watch has been issued for Hawke's Bay and Tararua from 1pm until 8pm on Sunday.
MetService said the thunderstorms will be accompanied by heavy rain at intensities of 40mm/h.
"The highest risk of downpours is about Central and Southern Hawke's Bay (from about Hastings southwards) and the Tararua District between about 1pm and 8pm Sunday," MetService said.
The forecasting agency said the thunderstorms should ease this evening before they return on Monday.
An orange heavy rain warning is in place for Mount Taranaki until 6am on Monday, with 150mm of rain expected.
MetService said rain should fall at peak rates of 25mm per hour on Sunday afternoon and evening with thunderstorms possible.
The ranges of Westland south of Otira will bear the brunt of the bad weather with an orange heavy rain warning in place until 6am Monday.
The forecasting agency said this area should expect 200mm of rain to fall at peak rates of 25mm per hour on Sunday afternoon and evening, with thunderstorms possible.
"Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous," MetService said about the areas under an orange heavy rain warning.
A heavy rain watch has also been issued for the Tasman ranges west of Motueka and Richmond and Bryant ranges north of Richmond township until Sunday evening and in the North Island the Tararua Range until 9am on Monday,
MetService warns these areas under a heavy rain watch could see thunderstorms on Sunday afternoon and evening.
The weather in New Zealand's big cities is also not flash for Christmas Eve. Auckland is expected to see patchy rain, turning to showers around midday with some expected to be heavy and possibly thundery before easing in the evening. But the temperature is set to be a warm 25C.
It's expected to be similar for Tauranga with some light morning rain and then a few showers from around midday that could be heavy.
Further south, Hamilton should see morning rain that will turn to showers towards midday with some of those heavy and possibly thundery. But it's expected to be warm and muggy with a high of 25C.
It's more of the same for Wellington with periods of rain, low cloud and northerlies becoming strong in exposed places this afternoon.
The pick of the main centres is Christchurch with a fine and sunny day expected and a high of 28C.
Further south in Dunedin, the rain will clear this morning but will remain cloudy before the showers return in the afternoon before easing the evening.
But while the weather might not be perfect for many the temperatures are still very balmy on Sunday morning.
The warmest places in the country at 7am on Sunday morning were Auckland, Whangārei, and Whanganui all reaching 21 degrees. Kaitaia, Whitianga, Tauranga, Napier and Palmerston North North were not far behind on 20C, according to MetService.
The forecasting agency said warm daytime temperatures are forecast for Sunday, especially for the east of the country once the clouds clear.
For Christmas Day, MetServices' severe weather outlook shows there is a "moderate confidence" of rain for large parts of the South Island and central, eastern and lower North Island, with thunderstorms also possible.
"There is moderate confidence that rainfall amounts could reach warning criteria over many areas from northern Fiordland to East Cape, as indicated on the chart," MetServices' severe weather outlook read.
"Over Westland and Mount Taranaki, however, the confidence is high. Warnings and watches are in force. Thunderstorms may also produce localised downpours over the northern North Island, mainly in the afternoon and evening."
Severe Thunderstorm Watch
Area: Auckland
Period: 7hrs from 1pm - 8pm Sun, 24 Dec
Forecast: This is an alert to the potential of localised downpours about the Auckland region this afternoon and early evening.
Area: Hawkes Bay, Tararua
Period: 7hrs from 1pm - 8pm Sun, 24 Dec
Forecast: Scattered thunderstorms are expected to develop over many parts of the northern and central North Island this afternoon and evening.
Area: Auckland
Period: 7hrs from 1pm - 8pm Sun, 24 Dec
Forecast: This is an alert to the potential of localised downpours about the Auckland region this afternoon and early evening.
Heavy Rain Warning – Orange
Area: Mount Taranaki/Taranaki Maunga
Period: 28hrs from 2am Sun, 24 Dec - 6am Mon, 25 Dec
Forecast: Expect 120 to 150 mm of rain near the summit, with lesser amounts at lower levels of the Mountain. Peak intensities of 15 to 25 mm per hour and possible thunderstorms during Sunday afternoon and evening.
Area: The ranges of Westland south of Otira
Period: 28hrs from 2am Sun, 24 Dec - 6am Mon, 25 Dec
Forecast: Expect 150 to 200mm of rain to accumulate. Intensities generally reaching 10 to 15 mm per hour, but may increase to 20 to 25 mm per hour Sunday afternoon and evening with possible thunderstorms.
Heavy Rain Watch (h2)
Area: Tasman ranges west of Motueka
Period: 26hrs from 8pm Sat, 23 Dec - 10pm Sun, 24 Dec
Forecast: Periods of heavy northerly rain and accumulations may approach warning amounts. Possible thunderstorms Sunday afternoon and evening. Note, rain may ease for a time Sunday morning.
Area: The Tararua Range
Period: 24hrs from 9am Sun, 24 Dec - 9am Mon, 25 Dec
Forecast: Periods of heavy rain and accumulations may approach warning amounts. Possible thunderstorms Sunday afternoon and evening.
Area: Richmond and Bryant ranges north of Richmond township
Period: 27hrs from 8pm Sat, 23 Dec - 11pm Sun, 24 Dec
Forecast: Periods of heavy northerly rain and accumulations may approach warning amounts. Possible thunderstorms Sunday afternoon and evening.
