After the hottest first three months of any year on record, much of the country may see a warmer-than-normal winter.
According to Niwa's just-released climate outlook, temperatures over the next three months were forecast to be above average in the north of the North Island.
That included Northland, Auckland, Waikato and the Bay of Plenty; elsewhere, temperatures would likely be above average or average.
But Niwa reported frosts and cold snaps would still become more common, with some cold snaps "possibly quite sharp".
And there was also a warning of big rain events: the result of lower than normal sea-level pressure but warmer than average Tasman Sea temperatures.
Rainfall levels in general were forecast to be above normal in the north of the North Island, near or above normal for the rest of the North Island as well as the east and north of the South Island, and normal or below normal levels were expected for the west of the South Island.
The balmy first three months of 2018 has given New Zealand its hottest recorded start to a year, with mean temperatures soaring to 1.75C above average over the period.
The scorching start came amid our hottest summer on record and the influence of a record-beating marine heatwave, and also included the sixth-warmest March ever observed in the country.
Niwa is due to release its climate summary of last month tomorrow, but Niwa meteorologist Ben Noll said April appeared to have broken the running trend.
"Basically the upper North Island was the only place during the month of April that had above average temperatures, so much of the rest of the country had either average or below average temperatures," he said.
"It would probably actually be the first month in some time where New Zealand's temperature for the country as a whole was near average."
But the record-hot start to the year would still rate 2018 as among the warmest ever so far.
"We've obviously got quite a bit of a cushion with that front part of the year being so warm, so it's going to take a couple of months to knock that back down the list."
With the marine heatwave having faded, Noll doubted whether 2018 would finish up as New Zealand's hottest.
"But, by the time July wraps up, we also don't expect the maps to be painted blue, meaning temperatures have been below average."
Soil moisture levels and river flows were expected to be near normal or above normal for all North Island regions and for the north of the South Island.
For the east of the South Island, river flows are also expected to be near normal or above normal, but with soil-moisture levels forecast to be above normal.
For the west of the South Island, soil moisture levels and river flows are forecast to be normal or below normal.
"The atmospheric circulation around New Zealand is forecast to be characterised by lower than normal atmospheric pressure through to July," Niwa reported.
"This will result in mixed and changeable air flows across the country.
"Warmer than average ocean waters that are present around the country are also expected to persist through the next three months, though forecasts suggest they will gradually weaken to the west of New Zealand."
Niwa reported that, over the past month, weak La Nina conditions had given way to an ENSO-neutral state, and while sea surface temperatures around the country had warmed over April, they remained slightly colder than average in the eastern equatorial Pacific Ocean.
Models indicated that this state would persist until early spring, until the tropical Pacific swung toward an El Nino.
In New Zealand, the system typically brings cooler, wetter conditions, bringing higher rainfall to regions that are normally wet, and often drought to areas that are usually dry.