"It may be okay for the tourists, but the farmers and the Fire Service won't like it," Dr Mullan said.
The Niwa outlook is not a forecast and looks only at likely weather patterns over the three months as a whole.
That meant it was not necessarily all bad news for people in the North Island, Dr Mullan said.
"It's probably not the best if you're camping.
"And if you have to make plans ahead of time, then it's not so good, you don't have a choice.
"But if you have a choice, keep an eye on the weather forecast.
"It's not going to be three months of doom and gloom, there will be breaks in it."
The previous Niwa climate outlook released in December predicted near-normal rainfall in much of the North Island with normal or above-normal rainfall in the north.
Dr Mullan said the La Nina had "bedded-in" and intensified over December.