Ms Griffiths said the cold weather followed an unusually warm start to the year.
"Temperatures in 2015 up until mid-April were running much warmer than usual for much of the country," she said.
"January was especially hot, and March was also unusually warm. April started exceptionally mild aside from a bitterly cold snap mid-month, where dropped temperatures for a short time and brought snow to southern New Zealand.
"After the mid-April cold snap, temperatures bounced back -- so much so that the first half of May was unseasonably mild, too."
Ms Griffiths said the temperature drop New Zealanders were now feeling was uncharacteristic for May.
"I think the sharp change from unseasonably mild at the start to the cold weather we have now has been a marked change."
Over the last week, much of the North Island has been hit by cold, wet and wild whether.
The upper North Island was lashed this weekend by rain and wind, resulting in power outages to about 3000 homes in the Auckland area.
Just under 800 of those homes in four West Auckland suburbs were without power for most of this morning, but all but a few had since had power restored.
Ms Griffiths warned the cold that New Zealanders were now feeling was set to stay.
Aucklanders can expect patchy showers throughout the week and strong southwesterlies, which would start to ease on Wednesday.
Wellingtonians can expect showers in the morning followed by a fine afternoon tomorrow. However, the capital would also be battered by fresh southwesterlies and patchy showers for much of the week.
For the rest of the North Island, strong cold southwesterlies and showers from Taranaki through to Northland were expected.
"By Wednesday things should improve slowly. Thursday and Friday look mostly dry for the North Island but there is a high risk of severe frost," Ms Griffiths said.
Snow and gales are expected to cover the South Island, accompanied by strong southwesterly winds. In some areas, the snow could reach sea level.
Warnings are in place for snow, severe gales and snow-covered roads both today and tomorrow.
The MetService said snow was also expected in parts of the North Island.
Snow warnings are in place for the the Napier-Taupo, Desert and Rimutaka Hill roads in the North Island, and Milford Road and the Lewis, Porters and Lindis passes in the South Island.
Drivers have been warned the blizzard-like conditions in the South Island will make travel difficult.
MAIN CENTRES FORECAST
* Whangarei - Heavy rain throughout the day accompanied by hail, strong southwesterlies with a high of 16C. Showers are expected to ease on Monday but the strong southwesterlies will remain.
* Auckland - Showers, some heavy with hail and strong southwesterlies, a high of 15C. Expect easing showers and strong southwesterlies on Monday.
* Hamilton - Showers accompanied by hale and fresh southwesterlies are expected for Hamilton with a high of 15C. Rain looks to ease slightly on Monday with southeasterlies.
* Tauranga - Fine spells with a few showers from afternoon, strong southwesterlies and a high of 16C. Expect increasing fine spells come Monday coupled with southwesterlies.
* Wellington - Fine with a chance of morning showers. A strong evening southerly to sweep the capital with a high of 13C. Showers look to easy on Monday but a strong southerly should remain.
* Christchurch - Cold southwest change in the afternoon coupled with showers and a high of 15C. Expect showers to ease on Monday and some snow for the Hill suburb and southwesterlies.
* Dunedin - Showers and snow by evening, southwesterlies rising to gale with a high of 9C. Expect snow showers to ease on Monday but strong southwesterlies should remain.
(Source: MetService)
