Australia 167 for 7 (Gardner 85, Mooney 39, Nensi 2-30, J Kerr 2-34) beat New Zealand 127 for 8 (M Kerr 32, Green 26, Wareham 3-18, Gardner 2-17) by 40 runs
New Zealand fell well short of the 168-run target, with Australia backing up a nervy two-wicket win in the series-opener on the same ground.
Australia, however, have not quite rounded into top form in their return to the format since winning the T20 World Cup triumph. They've been made to work by an inexperienced New Zealand, who have been gallant but outclassed.
The return of Phoebe Litchfield from injury did not ensure a stronger batting effort from Australia, who again threw away wickets. They slumped to 41 for 3 after being sent in to bat on a flat surface before Gardner's heroics lifted them to a total that proved well beyond New Zealand.
Spinners Gardner and Georgia Wareham combined for five wickets to ensure Australia retained the Rose Bowl trophy.
Gardner rescues another patchy Australia batting effort
Australia were hoping for a more resolute batting performance after almost botching a 163-run run chase in the opener. But they struggled in the powerplay and soon were in a hole after Ellyse Perry, Australia's best batter with 52 in game one, was deceived in flight by offspinner Nensi Patel and hit straight to long-on.
Despite her team's predicament, Gardner came out with all guns blazing to turn the game on its head. Her most belligerent stroke early was when she whacked Nensi over deep midwicket and onto the grassbanks, where a spectator claimed a well-judged catch. She dominated a 60-run partnership with opener Beth Mooney, who wasn't at her fluent best having been dropped on 3 and 25 in missed chances that proved costly for New Zealand.
Gardner hit another six off Nensi and, fittingly, reached a 29-ball half-century with a boundary. Even though wickets fell around her, she did not let up and she lifted Australia to respectability with a flurry at the death. She was run-out on the final ball of the innings having fallen short of her career high T20I score of 93.
Hardworking New Zealand attack can't finish the job
For the second straight game New Zealand's attack performed well against Australia's mighty batting-order but couldn't quite finish it off. They chipped away and have taken 15 wickets so far in the series, but will be frustrated to have come up short in both games.
Left-arm quick Bree Illing had set the tone in game one after trapping Georgia Voll with a wicked inswinger in the first over. She continued her stranglehold over Voll after another first-over dismissal. On this occasion Illing superbly delivered a short delivery that hurried Voll, who miscued straight up in the air for a return catch. Voll has not scored a run so far in this series.
Troubling the batters with accurate short-pitched bowling, Illing delivered a strong opening spell of 1 for 18 off 3 overs to help restrict Australia in the powerplay. She was well supported by seamer Jess Kerr, who backed up her three-wicket haul in game one by having Litchfield miscuing to mid-on.
But New Zealand's attack were helpless against Mooney and Gardner in the middle overs. Jess provided a much-needed breakthrough when she had Mooney holing out to long-on, capturing her fifth wicket of the series.
Kerr's terrific performance was soured by a loose 19th over, smashed for 20 runs off four balls by a red-hot Gardner.
Accurate bowling overcomes sluggish fielding from Australia
Australia haven't been at their clinical best in this series, including in the field. New Zealand skipper Melie Kerr was the beneficiary when on 18 she miscued left-arm quick Lucy Hamilton down the ground. Perry and Annabel Sutherland - both running towards the ball from opposite directions - did not communicate and almost collided into one another as the ball failed to go to hand.
Melie received more luck on 21 when Nicola Carey dropped a sitter at deep backward square that went through her hands and hit her in the face. But Australia's attack turned the screws in the middle overs to suffocate Melie and Maddy Green.
Eventually Melie succumbed to the pressure when Wareham claimed a brilliant catch at deep midwicket in a more familiar piece of fielding from Australia. But wicketkeeper Mooney on the next delivery was unable to hang on to a thick edge off Gardner to reprieve Izzy Sharp.
Australia will want to be slicker in game three.
New Zealand's batting lacks spark
New Zealand fought hard with the bat but just could not put the foot down. They never seriously challenged Australia's total, which had only seemed par in good batting conditions.
Georgia Plimmer starred in game one with 85 not out but could not repeat the dose after tamely chipping Kim Garth to mid-on in the third over. Flora Devonshire was again backed at the top of the order having in game one opened for the first time in her brief T20I career. She got through the powerplay, hitting a couple of boundaries along the way, but could never get going and was bowled by Hamilton in the seventh over.
New Zealand's batting went through a major lull that ultimately sunk them. Sharp and Polly Inglis did unfurl some inventive batting, but it was all too late. Selectors may ponder whether to promote them up the order for game three.