Yes, it really is hotter at night than usual - and there's only a brief day or so of relief in sight.

Auckland's overnight low temperatures have been above the city's January average of 15.5C on all except one night so far this year, reaching 19C on Friday night and forecast to be 18C tonight.

MetService forecasts one average night (15C) on Monday night, but seven more nights well above average over the following week.

It's the same story for most of the North Island. Wellington's overnight lows have been above its 13.5C January average and are expected to stay above average through the next week apart from two 13C nights tomorrow night and on Tuesday night.

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The National Institute of Water and Atmospheric Research (Niwa) says warm oceans are expected to keep air temperatures above average in the northern North Island and at or above average in the rest of the country through to March.

"Sea surface temperatures are currently much warmer than average around New Zealand and warmer conditions are forecast to persist. This is likely to be a dominant driver of New Zealand's climate," it said.

MetService forecaster Mark Bowe said a colder frontal system that hit the lower South Island overnight would bring cooler temperatures and heavy rain to Southland and the West Coast tomorrow morning.

"It will slowly move across the North Island on Monday morning, that will bring a period of rain," he said.

"Behind that is a ridge building in the South Island, so things will clear out in the South Island as the rain passes over the North Island."

Showers are expected to persist in eastern parts of the North Island into Tuesday.

Maximum daytime temperatures are also forecast to be above average, reaching 27C in Auckland today and a projected 26C tomorrow, compared with a January average of 23.1C.

Monday's forecast is 22C and Tuesday's is 23C, but the forecast is a return to 25C or 26C for the rest of the week.

Niwa said this week that 2018 was New Zealand's second-warmest year on record and the seas around the country were the hottest ever, with parts of the Tasman Sea as much as 6C higher than average at times.