Te Puke was also poised to be hotter than ever with an average of 15C, with Rotorua and Whitianga likely to have their second-highest temperatures, and Whakatane its third-highest.
"They are all running about a degree above normal," Dr Mullan said. "That's quite a lot, quite significant. We are seeing the stark reality of global warming."
Forecasters predict that most of New Zealand will have "normal" temperatures during the next three weeks, but eastern parts of the country such as the Bay of Plenty are likely to be warmer. This means Tauranga and Te Puke's temperature records are almost certain to be broken.
Mount Mainstreet manager Ingrid Fleming said Mount Maunganui would "absolutely" benefit from the warm weather.
"The beaches are going to be busy," she said. "When you're at the beach you pop into town for some shopping and a meal. Who doesn't enjoy sitting outside in the sunshine enjoying a nice meal and a tipple?"
This sentiment was echoed by Hamilton teacher Steve Horne, who spent yesterday morning swimming at Pilot Bay while his wife and daughter walked around Mauao.
"We enjoy the cafes and the swims," he said. "If you live in Hamilton, then any beach is good."
But the weather did not necessarily mean a windfall for businesses outside Mount Maunganui.
Hospitality NZ regional manager Alan Sciascia said the Bay was generally an attractive destination for holidaymakers over summer, and the weather did not have a huge effect on that.
"The Bay of Plenty is a place that people will come to anyway," he said. "It's only when there's going to be a terrible storm that people will cancel plans."
Indoor pools can see a drop in patronage during warm weather as people head to the beach instead. Bay Venues programmes manager Tina Harris-Ririnui said this happened at most Bay Venues pools except for the outdoor Memorial Pool.
Niwa data showed more than 70 places in New Zealand were likely to set new mean temperature records this year.
Globally, the World Meteorological Organisation said 2016 was set to be the hottest yet, with annual records falling for the third year in a row. Hot Year
Tauranga - warmest year ever recorded
Te Puke - warmest year
Whakatane - third warmest year
Rotorua - warmest or second warmest year