She said rain would also develop in the lower North Island, with a high of 19C predicted for Wellington.
There was also a chance the weather would clear in the lower North Island by midnight, Ms Ranger said.
There would be short periods of rain for those in the east of the South Island, but it was likely to be a mostly fine day.
"Those on the west of the South Island should get heavy rain, but that will ease to showers."
Weatherwatch analyst Philip Duncan predicted the driest places to be this New Year's Eve was Gisborne, northern Hawkes Bay, Coromandel Peninsula and South Canterbury.
He said a wall of rain would slowly move over western New Zealand on New Year's Eve, bringing heavy rain for the West Coast of the South Island and Nelson.
Those areas with the highest risk of rain were Taranaki, Wanganui, Manawatu, Nelson, West Coast and Waikato.
Christchurch and Dunedin would have a warmer, drier day although there was a chance of very light rain in Christchurch in the afternoon, Mr Duncan said.
He said it would be mainly fine throughout the North Island in the morning, before late afternoon rain would hit Taranaki and Wellington first, and then Manawatu and Wanganui, before the rain travelled eastwards into Northland and Auckland later that evening.
Looking ahead to the first day of the New Year, showers were expected for the northern parts of the North Island, however elsewhere it would be mainly fine, she said.
Things were looking mainly fine for the South Island, with some rain in Fiordland and cloud increasing in Westland.
Driest spots:
- Gisborne
- Northern Hawkes Bay
- Coromandel Peninsula
- South Canterbury
Highest risk for rain
-Taranaki
- Wanganui
- Manawatu
- Nelson
- West Coast
- Waikato
