Mr Adams has created a "best guess" map showing possible rainfall.
Some rain is expected around the bottom of the South Island, caused by moisture moving up from the south.
This will continue to spread on New Year's Day, as will the chance of rain up north as moisture from the tropics closes in.
Auckland, Northland, Bay of Plenty, East Cape and Waikato are forecast to be the warmest spots on New Year's Eve, with temperatures in the mid-20s, and party-goers in Queenstown and Wanaka shouldn't need to add extra layers of clothing.
From Manawatu to Nelson, highs will be in the early 20s, while the South Island's West Coast will be warm too, with about 20C in Westport.
Mr Adams said overall it would be a mild day.
Tara Allard, 3, of Papakura, plays with her father Matt in the water at Long Bay beach. Photo / Nick Reed
This week, rain will spread across the country on Wednesday, from west to east, but clear in many places early Thursday.
Mr Adams wasn't expecting any significant falls.
Meanwhile, today brings plenty of sunshine and warm temperatures in central parts.
Mr Adams said sea breezes were cooling coastal areas slightly, so today's hottest area is central Otago, with Alexandra and Twizel both pushing 30C.
