"We could end up with some pretty good weather on Christmas Day around parts of the country, and just a few coastal clouds."
Mr Duncan stressed the changeability of such long-range forecasts.
Yesterday, Government agency MetService's longest-range forecast was Friday.
Tropical Cyclone Evan, which caused destruction in Samoa and was due to hit Fiji overnight, was the only cause for concern, Mr Duncan said.
Models predicted Evan would reach New Zealand next weekend, with its remnants well east of the country by Christmas Day.
"Because (Evan) is going to have some effects a few days before Christmas, it's going to throw things a little bit out," he said.
By the time the storm reached New Zealand it would likely have lost its more damaging tropical characteristics, Mr Duncan said.
But it could bring heavy rain and winds to northern and eastern areas of the North Island.
"We won't be getting what Fiji is getting, or what Samoa has had. But we could be getting a system that produces weather warnings."
However, the weather should have cleared by Christmas Day, Mr Duncan said.
"There's nothing on the long-term forecast that indicates anything massive happening that could throw a spanner in the works.
"The only thing would be this low perhaps lingering a bit longer."
Without a dominant weather pattern such as La Nina or El Nino, the balanced weather pattern of this year's summer months was set to continue, Mr Duncan said.
"That lack of major extremes is going to continue on through the summer," he predicted.
"I don't see anything that's going to make it as negative as last year was."
