Out of runs through the WBBL and after a lean patch for Australia, Gardner hit her second half-century in three innings in Wellington on Monday.
It was the 27-year-old's highest score in an ODI for Australia, helping carry the side out of a mid-innings stumble of 4-39 that left them at 4-127.
Gardner hit nine boundaries and one big six over in her knock, as Australia aim to claim the three-match series 2-0 after the first game was washed out.
The allrounder had endured a tough trot with the bat before Monday's runs, after an underwhelming WBBL where she averaged only 12.87 for the Sydney Sixers.
Gardner had also passed 20 in just three of her previous 18 international innings until a fortnight ago, before hitting 50 against India at the WACA.
The reigning Belinda Clark Medalist then grew in confidence on Monday.
After Phoebe Litchfield's 50 helped lay the platform at the top for Australia in Wellington, Gardner powered the back half of the innings.
She combined in an important 63-run stand fifth-wicket with Annabel Sutherland, before taking the game on herself.
The right-hander was the only batter to truly get on top of Amelia Kerr (4-54), driving and pulling the legspinner for four in the one over.
Gardner also drove Molly Penfold for a four and then pulled her for six a few balls later, before helping take 21 from the 43rd over of the innings off Rosemary Mair.
Amelia Kerr (48) has taken four wickets for New Zealand in the final ODI against Australia. (Darren England/AAP PHOTOS)
At that point Gardner looked on for a maiden ODI century, after cutting, pulling and driving Mair for four boundaries in the one over.
But she was bowled by an Amelia Kerr wrong'un in the next over, stopping Australia from posting a score well in excess of 300.
Australia then lost their next three wickets quickly to be all out at the end of the 49th over, making the first time they have been dismissed batting first in an ODI since February 2019.
Kerr had earlier produced the ball of the series so far, bowling Ellyse Perry for 14 with a wrong'un that spun back between bat and pad.
Australia have already retained the Rose Bowl series ahead of next month's multi-format home Ashes series.
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