New Zealand were dismissed for 291 in 45 of their 46 rain-reduced overs. Their last seven wickets fell for 37 runs in 36 balls.
Captain Williamson was unfazed - and almost impressed - by the collapse.
"With wickets in hand, guys can come out and play aggressively, but Australia has an attack which can make striking difficult.
"I appreciated the selfless behaviour of the middle and lower order to try to score from ball one. It didn't quite come off. We would've liked more runs, but we still had a pretty good score.
"[The result] was a bit frustrating. The guys stuck at the task well and bowled aggressively to take those early wickets."
Williamson milked singles at will up to the 30-over mark, when he was 51 off 67 balls, before adding a further 49 off 30 balls.
Ronchi set the tone with 65 off 43 balls as a licensed aggressor. The innings went some way to justifying the decision to persevere with him as an opener, despite the failures of yesteryear.
"I was batting to chance my arm," he said. "I've been tasked with a similar role to what Brendon McCullum did [at the World Cup] and getting us off to a flyer.
"When Kane is at No3, it gives you more freedom."
The difficulty for New Zealand will be applying the same astute recipe against England on Tuesday in Cardiff.
The hosts look tournament favourites after chasing 306 for victory with eight wickets and 16 balls to spare against Bangladesh.
Two years ago, New Zealand lost their last ODI series in England 3-2. The hosts subjected the visitors to a revamped limited overs approach after failing to make the World Cup quarter-finals. The 3151-run tally was a record for a series of five matches or less at the time.
Eoin Morgan, Joe Root and Jos Buttler piled on the runs, with support from Ben Stokes and Alex Hales. That quintet remain in the England squad and have refined their strategy since.
Anything less than a win - including a tie or another washout - will leave New Zealand vulnerable to consecutive Champions Trophy pool exits.
An Australian victory over Bangladesh on Monday in London would increase the pressure. Rain is also forecast to pepper Cardiff - the venue for New Zealand's final two games - for much of the week.