Rotorua and the rest of the upper North Island are likely in for a warm and wet end to winter and start to spring, say weather forecasters Niwa.

Principal forecasting scientist at Niwa's national climate centre, Chris Brandolino, said although there were periods of unpredictable weather, climate scientists could generally give a pretty good prediction of the "flavour" of the next few months.

"The executive summary is: we're likely to have a warm, potentially wet end of winter and start of spring," Mr Brandolino said.

"There'll be cold snaps, there'll be some frosts, but that won't be the flavour, that won't be the theme ... a colder than average August, September and October is pretty unlikely."

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Mr Brandolino said lower pressure than normal to the west of New Zealand would likely bring weak northwesterlies, with little chance of any southerlies.

However, a wetter few months is on the cards.

"Significant low pressure systems" would likely bring higher than average rainfall. Mr Brandolino said there was only a small chance it would be below average.

He said long-term forecasts tended towards trends and were never perfect, but from the three possibilities - above average, near average, and below average - forecasters looked to show two possibilities as being more likely than the other.

"Rainfall's always a challenge ... a drier than normal three-month period is unlikely.

"I could tell you your marriage will be great for the next three months, it doesn't mean you won't get into an argument with your wife one night. That's the theme, but it'll happen."

Nationwide is fairly similar to the upper north - warmer temperatures likely, along with higher than expected rainfall.