"We had to really dig deep. You go into a dark place when you have a loss like that, and they really pulled it out and made everyone take notice, because a lot of people were writing them off," said Southby.
The win was built on a committed defensive effort right through the court, and a courageous and composed performance on attack, along with a couple of bold selection decisions.
Southby threw a curveball at the Australian defence by starting Bailey Mes in the unfamiliar position of goal attack, effectively swapping bibs with Te Paea Selby-Rickit.
But it was the inclusion of young defender Kelly Jury, who was sidelined from Wednesday's match with an ankle injury, at goal keep that proved the masterstroke.
Pitted against Diamonds captain Caitlin Bassett, Jury delivered a player of the match performance, forcing the Australian sharpshooter to the bench in the third quarter.
At 1.92m tall, Jury is just one centimetre shy of Bassett, eliminating the height advantage in the shooting circle Australia have enjoyed for so long. With the Australian attack unable to sight Bassett as clearly, they were hesitant to put the ball into the circle, upsetting their rhythm on attack.
There was much anticipation over the first meeting between New Zealand and Australia of the post-ANZ Championship era. Diamonds coach Lisa Alexander believed the lack of regular exposure to one another in transtasman league had amplified the rivalry between the two sides.
The split seemed to have worked in New Zealand's favour, with the Diamonds appearing to have forgotten how to play against the zone style of defence, committing basic errors that were out of character for the usually well-drilled Australians.
Bassett said she was disappointed her side were unable to adjust to the Ferns approach.
"I think it was just too many errors on our behalf. A lot of the mistakes were of our own doing and we didn't respond," said Bassett, who was delivered her first loss as skipper.
The Diamonds were their own worst enemies on defence, too, committing 84 penalties. Defender Courtney Bruce at goal keep was the worst offender, picking up 18 penalties before she was benched at three-quarter time after receiving an official warning from the umpires.
"I guess our Australian style is to be more on the body, but again, we just didn't adjust and that wasn't good enough," said Bassett.