Aucklanders are cranking up the air con to cope with the rising heat and humidity this week, causing a spike in electricity usage, Transpower says.
Temperatures soared to close to 30C this week, coupled with humidity, driving many to cool off in front of air conditioning units and powered fans.
Auckland residents used an average 1041 Megawatts (MW) between 9am and 5pm on Monday - up 7.7 per cent on the average of the previous two Mondays (966 MW), Transpower data showed.
Air conditioning, electric fans and fridges working overtime would account for almost all of the extra power use, Ari Sargent from Powershop said.
"It's been stinking hot and the normal reaction is to crank the air conditioner as much as possible which can really chew through power.
"Households also tend to leave air con and fans on all night when it's this hot, especially with babies or young children at home."
Aucklanders should plan for higher power costs this month if the hot and muggy weather continues, Mr Sargent said.
Forecasters say the heat and humidity and chance of thunder showers will persist throughout the weekend.
Yesterday, severe thunderstorms hit parts of the North Island and heavy rain last night caused localised flooding in South Auckland. MetService recorded 1164 lightning strikes across the upper North Island over a two-hour period to 6:30 last night.
Heavy rain caused flooding in Napier, where 21mm of rain fell in one hour between 10am and 11am, while a MetService weather station in Northland recorded 30mm in one hour. The most rainfall was in National Park, where 66mm accumulated in the 12 hours to 7pm. Taumarunui had 58.2mm in the same period.
The sticky, hot weather is due to hang around for the next few days, MetService meteorologist Claire Flynn said.
A moderate thunderstorm risk was in place for eastern parts of Northland, Auckland and the Coromandel Peninsula today, she said. The Bay of Plenty, Taupo and east of Waikato were also at risk.
The thunderstorms would likely develop in the afternoon and evening, Ms Flynn said.
A heavy rain warning is in place for Gisborne.
Overall, the upper North Island was "looking like another showery day, not as thundery as [yesterday] but there are still some risk areas".
"Still looking like some quite warm temperatures, particularly for the North Island," she said. "We're still looking at high 20s for the northern parts."
The humid air was causing the thunderstorms, she said.
"The thunderstorms are to do with all the humid air that's come down from the sub tropics and usually the air is quite unstable, which can cause these thunderstorms. It's kind of a similar story [today], but not as extreme. They'll ease off but will pop up again [in the] afternoon."
The South Island would start to turn fine, but would still see some rain around, she said. It would be a similar story for the North Island over the weekend but with a reduced risk of thunderstorms.
The South Island would have fine spells interrupted by the odd shower.
"It's the same air mass just sitting there so the general picture is not really changing too much, just less thundery than it has been [yesterday], that's all," said Ms Flynn.
Very humid, very warm weather will hang around the North Island, WeatherWatch also said.
"The next 10 days have highs hovering around 27C and 30C, which may not sound overly hot to some people but with humidity between 60 and 90 per cent for much of this time the 'feels like' temperature will be around the low to mid 30s," analyst Philip Duncan said. "Overnight lows will be tough for sleeping, with temperatures hovering around 20 at the coldest time, and with humidity often closer to 100 per cent at night making it feel like the mid- to late-20s for some."
Meanwhile, a mother and her baby narrowly escaped being dragged out to sea by large swells while on a walk at Mairangi Bay.
Takapuna woman Eliane Souza was at the North Shore beach after 4pm on Saturday when she saw a woman pushing a pram along the North Shore Coastal walkway during high tide, and took photos of the pair becoming engulfed in the waves.
"The waves came very quickly, then she ran to get to the end of the path."
Beat the heat without blowing your budget
• Keep blinds and curtains closed during the day.
• Keep windows open at night.
• Cook outdoors on the barbecue to keep the heat outside.
• Keep doors and windows closed when using fans or air conditioners.
Whangarei: Showers with a chance of thunder in the afternoon, a westerly breeze and a high of 28C
Auckland: Showers with a chance of thunder in the afternoon, a westerly breeze and a high of 28C.
Hamilton: Showers develop with a chance of thunder, a southwest breeze and a high of 29C.
Tauranga: Fine spells with afternoon showers and a chance some may be heavy, light winds and a high of 29C.
Wellington: Rain easing to showers in the morning, strong southerly eases, with a high of 17C.