A 10th over rain interruption reduced the match to 46-overs per side. The Black Caps were eventually dismissed for 291 in 45 overs on what seemed an even-paced pitch.
They lost their last seven wickets for 37 runs in 36 balls. The final six fell to catches. Josh Hazlewood finished with six for 52, the second best bowling figures in the tournament's history.
Further rain during the lunch break reduced Australia's target to 235 in 33 overs at a run rate of 7.12 per over as the Duckworth-Lewis Method made an early competition cameo.
England chased 306 for victory with eight wickets and 16 balls to spare against Bangladesh yesterday, proving teams can coast to targets requiring more than a run a ball as 50-over cricket morphs into a Twenty20 mindset.
Pre-tournament, the average run rate of teams playing one-day internationals in Britain since the 2015 World Cup was 6.14. That statistic looks like increasing in the coming days.
The hosts, with two points, are now in the strong position of only needing to win one more game to make the semi-finals. New Zealand and Australia share a point apiece.
Ronchi set the tone with 65 off 43 balls. The innings justified the decision to persevere with him as an opener, despite the failures of yesteryear.
His last ODI innings of such substance was clocking the world record No.7 score of 170 not out against Sri Lanka in Dunedin on January 23, 2015.
However, many of his interim innings demanded selfless swashbuckling in the death overs.
Australia made a dishevelled start, giving the New Zealand batsmen either room outside off stump to free their arms, or drifting onto leg stump to open up the onside.
Ronchi and Martin Guptill (26 off 22 balls) piled on momentum to post 40 before the end of the sixth over when Guptill was caught.
As a licensed aggressor, Ronchi continued his blitz. It was best exemplified in the 15th over when he took Pat Cummins for 14 runs and survived a dropped Mitchell Starc catch at mid-on amid the final four balls.
A Warner direct hit for a quick single saw Ronchi home by one television frame on seven, although umpire Richard Kettleborough never sent it for review.
On 24 at the end of the ninth over, he escaped an Australian run out butchery after a mix-up with Williamson. Wicketkeeper Matthew Wade eventually removed the bails with a sliding shoulder.
Ronchi's innings at a Brendon McCullum-esque tempo enabled Williamson (100 off 97 balls) and Ross Taylor (46 off 58) to sculpt a 99-run third-wicket platform which presented the middle and lower order with a chance to unleash. They failed to capitalise.
Williamson showed customary composure. He milked singles to rotate the strike where necessary, before upping the ante after the 30-over mark. The captain was 51 off 67 balls at that point, before adding a further 49 runs off 30 balls.
Glenn Maxwell proved a safe pair of hands in the field, taking four catches.
Inclement Birmingham weather in saw the covers on for long periods in the build-up. As a consequence, New Zealand opted to play Boult, Milne and Tim Southee as three specialist pace bowlers rather than bolstering the spin attack with Jeetan Patel on his Warwickshire home ground.
Mitchell Santner played as the sole spinner with Corey Anderson and Jimmy Neesham given the nod as all-rounders over Colin de Grandhomme because they are seen as stronger ODI batsmen.
