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The finalised election results, released at 2pm, show Ms Simmonds just pipping Labour candidate Liz Craig by 224 votes.
Ms Simmonds received 17,929 to Dr Craig's 17,705.
Neither candidate would call the race on election night, saying it was too close.
Dr Craig, an incumbent list MP, trailed former Southern Institute of Technology chief executive Ms Simmonds by 685 votes at the close of play on election day.
Ms Simmonds had 16,372 votes to Dr Craig’s 15,687. Her majority was significantly lower than retiring MP Sarah Dowie’s in 2017, when she beat Dr Craig by 5579 votes.
Labour's share of the party vote increased slightly with special votes counted, from 47.5% on election night to 47.7%. National's dropped from 30.3% to 29.7%.
Ms Simmonds said she was relieved and very, very grateful for the outcome.
She was in a meeting in Auckland when the results were released and only learned of her victory when she received a text message saying "you're in".
She believed the close nature of the Invercargill race was due to the "tremendous red tide'' that swept the country and delivered a huge victory to Labour.
"It was very, very hard to resist that.''
Her priorities for the term were the transition from Tiwai Point, farming regulations, and the polytechnic sector reforms.
Dr Craig described the result as incredibly close, and said irrespective of the outcome, she would continue to work hard for the area.
She learned of the results while attending an event in Invercargill this afternoon.
She thanked the community for its support, and also congratulated Ms Simmonds.
"I'll continue to work with her, as I have in the past, on things as we advocate for Southland together.''
She was planning to contact Ms Simmonds this afternoon.
The national results, which as expected were positive for Labour, were welcome, she said.
"I'm really pleased... I think it's just a really good result across the board.''