"They look like they are going to be the hottest days.
"Alexandra is due to get 35 and Wanaka will hit 24 and into Tuesday all centres will be heading in to the late 20s and early 30s and some could go a great deal higher," he said.
Christchurch is forecast to reach 34C on Tuesday.
It won't be as hot in the north but even some Wellington suburbs are expected to reach 31C.
"Across the Wellington region, Lower Hutt and Wainuiomata will be getting into the 30s on Monday. Wainuiomata is expected to reach 31C."
The north is continuing to suffocate under a blanket of humidity and temperatures that barely shift between night and day.
Auckland and Tauranga have endured balmy night-time lows of 22C and 23C - after daytime highs little more than 5C higher.
The warm tropical air will continue to flood across the North Island for the next four days, bringing little respite from the discomfort.
The month is shaping up to be the hottest in New Zealand history.
Niwa yesterday released preliminary data showing January's average maximum temperature was tracking well above record levels as the hottest month ever and the largest difference from average temperature since records began in 1909.
Train passengers sweltering
Wellington train services could face speed restrictions if the weather gets much hotter.
Trains on parts of the Wairarapa and Johnsonville lines have been limited to 40km/h this week.
Extreme temperatures are affecting tracks in several parts of the country.
KiwiRail spokesman Henare Clarke said the restrictions are a precaution against trains derailing and are put in place if the tracks reach more than 55C.
Meanwhile, more than 200 disgruntled train passengers on the Wairarapa line have signed a petition in protest at the sweltering conditions on the carriages.
The commuters say they're being treated "worse than animals", and complain about overcrowding, smelly toilets and a dysfunctional air conditioning system.
Petition organiser Greg Kerr catches the train daily and says the temperature onboard has become unbearable.
He says at around 32C, it's above the allowable threshold for transporting live animals.
Passengers said there's no air conditioning but Metlink said the units are struggling in the heat and a specialist will assess them next week.
Metlink said it was also closing curtains in the carriages to reduce the sun exposure.
Today's forecast
Auckland: Cloud breaking to sunny spells. Afternoon showers. Light winds. 28C
Hamilton: Cloud breaking to sunny spells. Afternoon showers with possible thunderstorms. 30C
Tauranga: Fine, morning cloud and afternoon shower. Light winds. 27C
Wellington: Cloudy, drizzle but sunny afternoon spells. Southerlies. 22C
Christchurch:Drizzle clearing to fine breaks. Noreasterly winds. 21C
Dunedin: Drizzle clearing to fine breaks. Noreasterly winds. 21C
