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Rangitata, which takes in Ashburton and Timaru, has gone to Jo Luxton, a list MP from 2017.
She told Mike Hosking on Newstalk ZB that she could feel the mood changing in the electorate.
"The feel on the ground has been so different this time around. I worked damn hard, damn hard, over the last three years."
Luxton's victory comes after the resignation of disgraced former Rangitata MP Andrew Falloon in August.
Rangitata electorate results:
Jo Luxton (Labour) - 18,876
Megan Hands (National) - 15,392
Hamish Hutton (ACT) - 1220
It follows a theme also seen in the greater Christchurch electorate's last night.
Nearly all of the seats in the area went to Labour.
The party will retain all the seats it held before while adding a couple of fresh faces to its Christchurch contingency.
In what is one of the biggest upsets for National, senior MP Gerry Brownlee lost his seat in Ilam to newcomer Sarah Pallett from Labour.
Brownlee entered Parliament in 1996, winning the Ilam seat and has held it ever since.