New Zealand all-rounder Scott Styris was delighted after taking full advantage of his unexpected opportunity in the Chappell-Hadlee series opener.
Styris only made the Black Caps' starting XI in Napier after the late withdrawal of captain Daniel Vettori with a neck injury.
Ross Taylor led the side in Vettori's absence and hit 70 from 71 balls to set up a thrilling finale.
And Styris was the hero with a quickfire 49 not out as New Zealand chased down a target of 276 with four balls to spare to go 1-0 up in the five-match series.
"To get out there and have a chance to win it I was obviously ecstatic," Styris told Radio Sport.
"The boys have worked so hard and it was good to get them a win.
"I was a little bit rusty to start with. I haven't had an innings for a couple of weeks so it took me a little bit to get going but the wicket out here was top drawer and great for batting, and thankfully once the power play came I was able to get my shots away."
The loss was Australia's first in 13 matches and came on the back of a super-over defeat in the second Twenty20 in Christchurch on Sunday.
"This is a big series. This defines our summer. I think Daniel Vettori has already said that," added Styris.
"To win back-to-back games, albeit in different versions of the game, is a good start for us.
"Hopefully they know they've got a contest now and we're here to play."
Australia skipper Ricky Ponting felt his side's total of 275-8 after opting to make first use of a good batting wicket at McLean Park was never going to be enough.
Only Michael Hussey (59) got a half-century, while Shane Watson (45), Ponting (44), Cameron White and James Hopes (both 33) all made starts but failed to push on.
"I probably felt all night we were just a little bit behind," Ponting told Sky Sports.
"I didn't think 275 was enough runs on that wicket. The ground is so small and the wicket was very good right through the game.
"Full credit to New Zealand I thought the way they bowled and fielded was terrific. Ross Taylor played a fantastic knock and Scotty Styris got them over the line so they were a bit too good today."
Ponting was confident his players, who have swept Pakistan and West Indies aside in the 50-over format this summer, would take the lessons from the loss on board ahead of the second game in Auckland on Saturday.
"It's never nice to lose a game, but any loss you have, you always learn more from a loss than a win, so we've got to look at what we did wrong tonight and make sure we train better leading up to the next game," he continued.
"That's the important thing now. I've just had a quick chat with the guys in there about the way we executed things tonight and just some of our shortcomings. It's up to the group now to respond well."