Australia paceman Pat Cummins has fired a warning at the nation's Cricket World Cup rivals, saying the champions may have "clicked" but are yet to put in a complete performance at the global showpiece.

Aaron Finch's Australia booked their place in the semi-finals early with a comprehensive win over England and carried on the form with another convincing victory over New Zealand at Lord's on Saturday.

A relaxed Australia will head into their final round robin match against already eliminated South Africa in Manchester on Saturday with the benefit of a week's rest before bigger tests await.

"I think just everything's clicked, the batting, the bowling, our game plan is the way we've wanted to play," Cummins told reporters.

"Even just everyone being comfortable in their roles and comfortable around each other. I think it takes time but everyone's in a really good place.

"The thing we have spoken about, we have managed to scrape home wins without really putting through a complete performance," Cummins added.

"With each game, we have got a little bit better."

Led by pace spearhead Mitchell Starc, Australia's quicks have bowled superbly throughout the tournament but have gone up a level in recent matches, skittling England's vaunted batting lineup for 221 before routing New Zealand for 157.

Jason Behrendorff's promotion to open the bowling with Starc, with Cummins switched to first change bowler, has been key to the improvement.

Cummins has only nabbed one wicket from the last two matches but has proved highly effective at keeping the batsmen pinned down during the middle overs.

He captured veteran Ross Taylor's wicket and conceded a miserly 14 runs from his six overs' work against New Zealand.

"I think it's a massive change," he said of the shift from new ball to first change bowler.

"I think it's a lot different to say red ball cricket where the ball kind of stays hard and there might be some swing on offer.

"I think it's kind of outside of those first eight to 10 overs, there is no swing, the ball is not as hard, normally the batters are set.

"Jason and Starcy have been brilliant up front. My role is to just try and hold them a little bit and hopefully they take a risk. It (feels) like I have done that job pretty well."

The match against South Africa will be a dead rubber but also the first that opener David Warner and former skipper Steve Smith have had against Faf du Plessis's side since they were suspended for ball-tampering during the Cape Town test in March, 2018.

Warner and Smith, whose one-year ban expired at the end of March, have been jeered at stadiums throughout the tournament but have not let it affect them, with the former leading the tournament's runs list with 516 and the latter chipping in 287.

Cummins did not feel the ball-tampering history would add much significance to the Proteas game.

"I know Dave and Smithy have played quite a bit with (South Africa's players) in IPL and played heaps of times before so I wouldn't have thought so," he said