Heavy rain and gale-force winds could deliver the dramatic finale of a sweltering January poised to be our hottest on record.

What could become the season's first tropical cyclone might also bring more of the hellish humidity that's sent thousands scrambling to swimming spots across the country this month.

The forecast follows a scorcher of a weekend, where the mercury was pushed above 35C in some parts of the country.

MetService forecasters are now tracking a system in the Coral Sea that could turn into a tropical cyclone, before tracking toward New Zealand at the end of the week.

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"This is quite unfathomable at this stage seeing that we are predicting temperatures between the mid to late 30s for most of the country for the beginning of the week," MetService meteorologist Kyle Lee said.

Predicted for main centres over Monday and Tuesday, along with potential hunderstorms, were respective highs of 29C and 27C in Auckland, 30C and 27C in Hamilton, 27C and 25C in Tauranga, 27C and 26C in Wellington, 25C and 32C in Christchurch and 27C and 30C in Dunedin.

But things could take a turn as early as Wednesday, with the potential for "a lot higher rainfall figures that we have seen for a while" and accompanying gale-force winds, Lee said.

Source / MetService
Source / MetService

WeatherWatch head forecaster Philip Duncan said what remained of the tropical cyclone could also keep things muggy.

"Think of a tropical storm as a candy floss machine - the centre of the low is the stick and as it spins in the tropics humid, hot, air attaches itself to the low, wrapping all around it," Duncan said.

"When the low tracks south it takes all this humid weather with it and can transfer tropical weather to New Zealand."

This could happen on Thursday and Friday, before a southerly behind the low cleared things up briefly, Duncan said.

But New Zealand may be about to deal with a double dose of tropical trouble.

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It was possible that the first week of February could see another tropical storm to our north.

"The second one is too far out to lock in but models are suggesting it could impact New Zealand around Waitangi Day - although it's still a very long way out and could miss us, but may bring dangerous beach conditions if it does eventuate and track nearby."

Duncan said the beginning of February opened up the doors to the tropics north of New Zealand - and could mean very heavy rain in both islands along with heat and humidity, especially for the North Island.

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Posted by MetService New Zealand on Saturday, 27 January 2018

"At this stage neither low is forecast to hit New Zealand as a storm - but it's just highlighting the fact that the tropics are now becoming incredibly active and all it takes for New Zealand to be impacted by more hot and humid weather is for high pressure to leave our area and allow tropical lows to drift south towards us."

For many Kiwis, January had already proven hot and humid enough - it hit 35.2C in Wanaka on Saturday and 32.5C Sunday.

Climate scientist Jim Salinger said this summer was looking set to eclipse that of 1934-35 for consistent, record-breaking heat.

Comparisons to date show the period from November through January has been running about 0.3C warmer than the 1934-35 event, when New Zealand baked in drought conditions that lasted from the beginning of November to the middle of February.

Salinger said both events had been influenced by the atmospheric effects of a positive Southern Annular Mode, but this time, the background impacts of a warming climate were likely also in the mix.