Wairoa in the Hawke's Bay hit 29.6C at 2pm today, comfortably breaking the North Island's previous heat record for September.
NIWA meteorologist Ben Noll called it "record-smashing".
The previous record was set in September 1955, when Hastings recorded 27.7C.
And today's temperature came close to setting the new record for the earliest 30C reading during spring.
"The 3pm observation that came in was a little bit lower," Noll said. "It looks like the maximum is going to be 29.6C in Wairoa today.
"[But] we surpassed the previous record by a very wide margin of 1.9 degrees, so that would qualify as being record-smashing rather than just record-breaking, I would say."
It comes with over a week left in the month.
"Looking ahead here, there is another warm day in the eastern North Island tomorrow," Noll said.
"But I don't think we'll quite get there. It's gonna be warm, I don't think we'll get to 29C and change again."
After tomorrow's heat, a southerly is set to move up the country over the weekend, bringing cooler days.
"The record warmth will take a break, but it won't be the last time that we're probably going to see it," Noll said.
"As we look ahead to early October, there may be another opportunity to get some air coming across the Tasman from Australia, which is what is driving the current outbreak of warmth, a flow of air that's coming all the way from northern Queensland in New Zealand.
"So while we will get a little break from the heat in the coming week to 10 days, it's probably gonna come back in early October."
The records are charted all the way back to the 1850s at some climate stations.
"It's pretty significant, you're talking about 170-plus years," Noll said.
The South Island's record September temperature of 31.1C was set in Canterbury's Lincoln in 1924.
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