"Nevertheless, we're about to get into a pattern that favours high pressure, with increasingly warm temperatures around New Zealand."
Fine weather was forecast for Auckland from Tuesday, with daily highs for the week hovering between 22C and 24C.
The outlook was the same for Whangārei, Hamilton and Tauranga, with highs of 25C, 26C and 25C respectively forecast at the end of the week.
Hastings could reach 27C mid-week - while New Plymouth and Palmerston North would more likely see temperatures reach the early 20s, and Wellington could see highs of 18C and 19C - albeit with sunshine.
In the South Island, Christchurch could reach 30C on Wednesday and Saturday, with plenty of fine weather in store for northern spots like Nelson and Blenheim.
Further south, in Dunedin, Queenstown and Invercargill, the week's weather was likely to be patchier.
"Next weekend looks really toasty, with widespread warmth, but especially in the east of both islands," Noll said.
"Overall, this picture represents what we alluded to in our seasonal outlook - that we'd have potential for a nice settled spell in the middle of December.
"That'll largely be welcomed with open arms, but in some parts of the country that are still battling with soil moisture issues, it might not be so nice."
While Christmas Day was too far away to call, Noll said there was good potential for building warmth and humidity over the week before.
"That's going to make it feel really uncomfortable for some people."
Noll said forecasters were also keeping a sharp eye on a tropical cyclone expected to develop between Fiji and Vanuatu over coming days - which could influence our own weather later in the month.
"So we can say that, while warmth and humidity are a pretty good bet leading up to Christmas, those rainfall patterns are really the big question. It will all depend on whether we receive any of the remnant moisture from that cyclone."
Noll said the untidy outlook - and the changeable weather over past weeks - could generally be blamed on a La Nina climate system.
Traditionally, the ocean-driven phenomenon brought warmth everywhere over summer, but with wetter weather around the North Island's northeast, and dryness to the south and southeast of the South Island.
Its influence came through clearly in Niwa's three-month summer outlook, picking a mix of above-average temperatures, high humidity, near-to-above normal rainfall in most regions, and the potential for sub-tropical flows to bring big downpours.
"At this point, La Nina is front and centre," Noll said.
"While there are always going to be ups and downs in any given season, we're expecting northeasterly winds - a hallmark of La Nina - are going to become more prominent here over the next couple of weeks."
Noll said New Zealand's coastal waters were also being closely watched, with "marine heatwave" conditions having already formed around northern areas.
"We could see quite an increase in sea surface temperatures here over the next two to three weeks."
While balmier surf was good news for beachgoers keen for a Christmas-time dip, the past few years had shown these heatwave events could be damaging for our marine and alpine environments.
The year was expected to go down as the seventh-warmest on record - continuing a clear trend driven by climate change.
