Southwell slotted a one pointer with eight minutes left to play before having a hand in the final play of the game for NSW centre Jess Sergis' late try in front of 20,179 at McDonald Jones Stadium in Newcastle on Thursday night.
Halfback Southwell's cool head - she had missed her first attempt a few minutes earlier - has assured the Blues are just one win from claiming a second-straight series.
"She had an opportunity earlier and got charged down ... but then she got another opportunity and iced it," said NSW coach John Strange.
"The fact that she did that after the first time is a testament to who she is."
Strange's side now face away trips to Brisbane on May 14 and the Gold Coast on May 28.
"We went up there last year for game one, and the girls played really well so for me it doesn't really matter where we play," Strange said.
Neither side was fluid - there were 19 errors across the 70 minutes - but as far first Origin hit-outs go this still felt high stakes.
The Maroons had to contend with losing two players to head injury assessments (HIA), including Mackenzie Weale in the first minute, but were in the fight right until the end.
Weale's replacement Otesa Pule, one of the New Zealand representatives cleared for Queensland because of the loosening of the Origin eligibility rules, took the field.
Sydney Roosters prop Pule made an instant impact by charging over from close range under the sticks to give the Maroons a second-minute lead.
Lauren Brown nudged the conversion and the visitors held on to lead 6-0 at the break, the NSW attack struggling for spark except for a few cavalier runs from winger Jaime Chapman.
The Blues had all the territory and possession in the opening 10 minutes after halftime and turned to brute force to get off the mark when prop Ellie Johnston powered through some heavy traffic to score.
Southwell converted to draw the sides level.
Queensland's Keilee Joseph dropped the ball with the line beckoning and then went off for a HIA with 12 minutes left after completing a tackle on Tiana Penitani Gray.
Given the Maroons had lost Weale they were permitted to activate their 18th player in Destiny Brill.
Brill charged down Southwell's first attempt at a field goal before the playmaker made no mistake with her second.
As the siren sounded the ball was flicked wide to Sergis who cut back against the grain to wrap up victory for NSW.
"I'm extremely proud of the girls, but it's a loss and we came down to win and we didn't get that, so that's hurting us," said Maroons coach Nathan Cross.
"We missed a chance ... but we can see where we can win it next time."
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