Parts of New Zealand are bracing for a battering of rain and thunderstorms today as a tropical cyclone strengthens near Australia.

MetService has placed several warnings and watches across both islands with torrential rain expected to hit Gisborne and Westland this evening and through to tomorrow.

A red warning was issued for Hastings and Rangitikei districts at 4.30pm today after the MetService weather radar detected severe thunderstorms near the Kaweka and Kaimanawa ranges.

The thunderstorms are expected to lie over these areas at 5.30pm and will be accompanied by torrential rain, MetService said.

“Torrential rain can cause surface and/or flash flooding about streams, gullies and urban areas, and make driving conditions extremely hazardous.”

In Gisborne, rain is expected to pour from 9pm tonight with peak rates of 20 to 30mm/h on Monday morning.

MetService meteorologist Claire O’Connor told the Herald the rain comes from a lower pressure system which is descending on New Zealand from the north.

“Given the way the winds go around, it will direct the rain towards the eastern coast so that’s where those warnings come into play there.”

Despite the rain, it’s a sweltering day for much of the country with Gisborne and Auckland currently sitting at 27C.

The hottest region so far is Masterton which has hit 30C, and Napier, Palmerston North and Hamilton close behind at 28C.

In the South Island, it’s slightly cooler with most regions sitting between 20C and 25C. The coldest place in the country is Stewart Island coming in at a balmy 18C.

In Tongariro National Park, lightning has already been detected with the potential for severe thunderstorms to develop in the coming hours.

Taumarunui, Taupō and Taranaki are all under a severe thunderstorm watch until 9pm tonight with the potential for heavy storms to develop.

Further south, there’s a watch of heavy rain for Westland which O’Connor said is caused by a front moving up from the south.

“There’s a bit of precipitation that will impact the West Coast - but it’s fast moving so it won’t have as much impact as the weather the other day.”

On Friday the West Coast was under a state of emergency due to torrential rain that caused rivers to rise rapidly and forced the closure of State Highway 6 between Haast and Hokitika.

O’Connor said despite more rain being forecast, it wasn’t all bad news - a much-needed break in the high heat and humidity is also forecast to drift across the country in the coming week.

“With this feature moving up the country, these cooler south-[westerlies] go right across the country flushing out the humidity and the heat as well.”

Overnight temperatures could drop into single digits in the South Island this week and Invercargill is looking at a chilly Wednesday - with a high of 13C forecast.

Further north it’s a similar story with overnight temperatures expected to dip to half of their current temperature.

“The highest [temperature] I can see overnight on Wednesday is 15 degrees,” O’Connor said.

Tropical cyclone strengthening

O’Connor said the tropical cyclone that is strengthening over the Coral Sea is looking like it will hit over the Ditch before it makes landfall in New Zealand - if it hits at all.

The cyclone is forecast to hit Australia on Monday, but O’Connor said it’s difficult to know whether it will strike New Zealand, and how severe it will be if it does.

“It’s going to hit Australia, particularly Queensland but in terms of for New Zealand, it’s still quite far out that a lot of uncertainty and in the models that we can’t say for sure what impact would be for New Zealand if it did come our way.”

Forecasters are closely tracking potential for a tropical low over the Coral Sea to form into a cyclone system, with Australia’s Bureau of Meteorology currently giving a “high” chance (or 55 per cent) of it reaching cyclone strength by early next week.

Niwa meteorologist Ben Noll said that, among other favourable conditions for cyclone development over coming weeks, surface temperatures in the Coral Sea were running warmer than average.

“Basically, west of New Caledonia, extending all the way to the coast of Queensland, we’re seeing moderate to strong marine heatwave conditions.“

Up in the Coral Sea, meanwhile, the marine heatwave could increase the potential for a formed cyclone system to intensify, while contributing more moisture.

“I’d agree it’s too early to say what we can expect with this one,” Noll said of its implications for New Zealand.