Previous to Thursday, the August record was 20.1C set in 2009.
Whanganui also had a record-breaking overnight temperature on Thursday, reaching the warmest minimum for August of 16.4C.
This was 2.6C higher than the previous record of 13.8C set in 2015.
"What climate change does is it makes a significant weather event more likely to occur and more intense. It also increases their frequency," Brandolino said.
"So this same event, if you were to put it into a time machine and the same natural conditions, in 50 years that sort of event will produce something more intense and will be more likely to occur."
He said this was due to greenhouse gases and what that does to the atmosphere.
"They increase the temperature of the Earth, when you have a warmer Earth, water is more likely to be in its vapour state and water vapour is the fuel for rain, and more warmth."
"Obviously there's huge impacts, and as the average temperature goes up, the extremes do as well."
Niwa climate scientist Dr Daithi Stone said this was because the extremes were more relevant to today's average temperatures.
"When we break records, we break them at a surprising amount."
He said all of the warmest records in New Zealand had been in the past several years.
"This is a consequence of climate change, and its most obvious manifestation is the warming of the planet. We see it in average temperatures, and in the extreme temperatures.
"There are lots of impacts on the planet, and in New Zealand, we're still figuring a lot of that out. One visible one is the retreat of the glaciers and snow on mountains."
He said people can certainly expect to see warmer temperatures over time.
"Warmer air can hold more water, and the warmer it gets the bigger these dumps of water could be.
"The crazy amount of rain across the country we've had is a little more tricky."
He said the recent weather system stalled, and they're not sure if that is something that can be seen more as time goes on.
Metservice meteorologist Dan Corrigan said the weather set-up has basically been just right to get the unusually warm temperatures.
There's been a persistent northerly wind, and good cloud cover which means the heat doesn't escape to space as easily, then there's also the warmth coming off the mountains.
"All those factors combined have been why it's a big week in terms of the record books."
He said in general temperature records were being broken more frequently.
"We can't say it was directly caused by climate change, but due to the effects of climate change we're likely to see events like this happen more often."
While the warm air coming down from the north brought the warm air to Whanganui, it's also a contributor to why we've had the heavy rainfall in Nelson, the Tasman and Malborough last week as well.
"That's because warm air is filled with moisture and able to hold more water vapour."
He said Whanganui didn't experience the heavy rainfall because it's more sheltered by the mountains.
Springvale Garden Centre manager Gareth Carter said the unseasonable weather was noticeable.
"Over the last week you can definitely see growth in plants has kicked off, you can see it in the nursery here," he said.
"A lot of growth has actually been delayed this winter. Anecdotally it seems we had less sunshine, so we've actually been behind.
"But then there's been less sunlight, which definitely has slowed the growth because you don't have that same rate of photosynthesis happening.
"Things need the light and warmth of the sun, particularly for vegetable gardens.
"It's quite random, winter has been greyer and wetter than usual, but not colder than usual."
He said there were a strange lack of frosts.
Whanganui Federated Farmers dairy chair Andrew Pearce said last week's heat was noticeable in grass growth.
"Last week helped us increase soil temperatures and grass growth," he said.
"Like everything, it's complex and it's very relevant to the time of year. A heat wave in summer is very different to a heat wave in winter."
"Cows can get under heat stress when temperatures climb into the late 20s and heat stress means they just don't function as well and they might not eat as much so we have to continually evolve and have things like more shade.
"I've been farming for 30 years and weather patterns have definitely changed over that time.
"As farmers, there is no normal anymore and when you're dealing with natural elements the variable is not up to you, unexpected changes could throw things off majorly.
"You never know what you're going to get, week to week, so we need to be prepared for bigger variations."
