Today is gearing up to be the hottest day of the year with the east coast of the South Island sweltering in temperatures above 30C and inland areas expected to tip 40C.

As the heatwave sends the mercury rocketing for days on end across Central Otago the entire country remains under a super-charged heater with many parts of the North Island continuing to experience balmy days in what is likely the country's hottest month ever.

By 2pm, Waiau Hamner in the Canterbury region were both already sitting at 36.4C.

Cheviot in the Hurunui District wasn't far behind on 36.3C and Clyde in Central Otago was 36.2C.

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The ballistic temperatures are set to continue with the only main centres in the South Island to miss out on 30C or higher will be Nelson, Westport, Hokitika, Oamaru and Invercargill.

Residents of Alexandra and Twizel are expected to sizzle on 35C but MetService said a weather station in the area may reach the late 30s or even 40C as the summer heatwave continues to see a swathe of records rewritten on an almost daily basis.

Yesterday Cromwell was the hottest spot in the country reaching a searing 36.6C and equalling its hottest ever January temperature since records began.

Niwa warned people to expect warmer conditions still across the South Island today.

The last time it reached 40C was in 2011 at Timaru Airport.

The hottest ever day happened on February 7, 1973 when northern and eastern regions of the South Island experienced a phenomenal heat reaching a record 42.4C in Rangiora. Nine out 10 of the country's hottest temperatures were recorded that day.

Medical authorities are warning people to keep out of the sun and avoid extreme physical exertion in the heatwave conditions.

People are also being told to drink at least 2l of water a day to stay hydrated.

Masterton is expected to be the hottest city in the north, reaching 33C, and Taumarunui is also looking to reach 30C. By 11am Masterton was 29C.

Auckland is in for another brilliant day with 28C - the fifth consecutive day the city of sails has nudged the 30C mark.

Niwa this morning revealed the waters surrounding New Zealand were also significantly warmer than this time last year. Coastal sea temperatures ranged from 24C to 26C around Auckland and an even more tropical 26C to 28C in the Firth of Thames.

Record breaking month

Today climate scientist Dr Jim Salinger said for the first 29 days of the year the average temperature across the country sat at 2.9C above average. With a further two days to go and even higher temperatures in store it was likely to be up to 3.1C above average.

He said it was the warmest month on record since reliable records started in 1867.

People told to take care in hot conditions

Those looking after the most vulnerable in our community were told to take care to ensure temperatures were cool enough whether it was in a workplace, school, rest homes or sporting and cultural events.

The co-convener of the country's climate and health council Dr Alex Macmillan said the heat posed serious health risks to the youngest and oldest in our communities and to those suffering from medical conditions.

"Take care as the heat rises especially if you're elderly, pregnant, or already have a medical condition. Babies and children are also more at risk with rising heat, while healthy adults who work outdoors are also especially vulnerable," said Macmillan.

"This is a good time to stay out of the heat, cool yourself down, drink plenty of cold non-alcoholic drinks and look after others, especially if they are vulnerable."

He said even short duration heatwaves could lead to a spike in heat strokes, heart and lung disease and deaths.

Schools told to keep watch on students

The Ministry of Education says the decision to stay open in the current extreme heat is up to individual board of trustees who are responsible for health and safety of all in schools.

The ministry recommends the indoor temperature ranges between 18-25C for much of the year.

Head of Infrastructure Service Kim Shannon said where this was proving difficult schools were told to pull blinds in windows facing the sun, keep windows and doors open to allow a breeze through classrooms, switching on fans and make use of shaded designated outdoor spaces.

Shannon said parents with concerns about the environment their children was learning in was encouraged to contact the school in the first instance.

More bad weather just days away

Meanwhile, an ex-tropical cyclone is closing in on New Zealand with wild weather set to hit the country on Thursday.

Weatherwatch.co.nz said the approaching storm would hit the South Island at the same time as a cold front from the Southern Ocean reached the far south. The storms would collide bringing very heavy rain that threatened to flood Fiordland.

This morning MetService issued a heavy rain warning for Westland and Fiordland.

In some parts of Westland a phenomenal 380mm of rain is expected to fall between Wednesday night and Friday morning. The two day deluge is expected to lead to flooding and slips with streams and rivers to rise rapidly.

There are possible damaging gales for parts of the South Island, especially in west and northern regions.

Weatherwatch.co.nz said the bad weather would sweep north with torrential rain, thunderstorms and flash flooding to isolated parts of the North Island.

The higher than usual king tides coupled with storm surge, strong winds and heavy rain meant the West Coast was at risk of coastal flooding.

strong>Today's forecast highs

Whangarei 26C
Auckland 27C
Tauranga 25C
Rotorua 26C
Napier 28C
New Plymouth 27C
Palmerston North 31C
Wellington 26C
Nelson 28C
Blenheim 34C
Christchurch 32C
Greymouth 25C
Ashburton 33C
Timaru 31C
Dunedin 31C
Alexandra 37C
Wanaka 33C
Queenstown 31C
Invercargill 30C