Guptill and Williamson added 131 runs for the second wicket.
After losing opening partner Tim Seifert (3) cheaply, Guptill added 131 runs for the second wicket with skipper Kane Williamson, who smashed 53 off 35 balls. Later, Jimmy Neesham struck 45* off 16 balls to propel the Kiwis to a massive 219/7 in 20 overs.
WATCH - Jimmy Neesham pulls off a reverse ramp shot; Wasim Jaffer reacts with a meme
Chasing the target, Australia fell four runs short despite Marcus Stoinis’ 37-ball 78. Neesham took two wickets in the final over when the rampant visitors needed 15 runs for victory while left-arm spinner Mitchell Santner returned with the figures of 4 for 31.
After the win, Guptill said partnership with Williamson set the game up for New Zealand.
"The beauty about cricket is that it doesn't matter about what's happened in the past. You move forward. You go on. I was lucky enough to spend a bit of time at the wicket today and get a few runs, but having Kane [Williamson] out there in that partnership really set the game up for us," stuff.co.nz quoted Guptill as saying.
With this victory, Black Caps took a 2-0 lead in the five-match T20I series and Guptill's next target is to claim an unassailable lead by winning the third game.
He also hailed the Black Caps bowlers for keeping the Australian batsmen at bay.
"I'm looking forward to hopefully going 3-0 up in Wellington. Our guys held their nerve. Tim and Trent's last overs were really clutch, and they bowled fantastically to keep Australia at bay," Guptill remarked.
The third T20I is slated to be held in Wellington on March 3.
(With ANI inputs)