Usman Khawaja and Alex Carey both hit fifties as they shared a century stand that saw reigning champions Australia recover from a top-order collapse in their World Cup match against New Zealand at Lord's on Saturday.

The left-handed pair came together with Australia having slumped to 92-5. 

But when Carey fell for 71, chipping New Zealand captain Kane Williamson's off-spin to Martin Guptill at extra cover, Australia were 199-6 with seven overs left.

Wicketkeeper Carey faced 72 balls and hit 11 fours, the best arguably a superb straight drive off Trent Boult.

Australia, who beat New Zealand by seven wickets in the 2015 World Cup final, had already qualified for the last four out of the 10-team round-robin phase before captain Aaron Finch won Saturday's toss.

His decision to bat first on a used pitch -- the same track on which Australia beat England by 64 runs at Lord's on Tuesday -- was understandable given the sunny conditions but a worn surface provided some variable bounce and sharp turn.

The form of Australia's first-wicket pair has been a key factor in much of their success at this tournament, with Finch and David Warner having scored 657 runs together in eight matches, including Saturday's fixture, at an average of 82.12.

But it was a different story against the Black Caps, with both Finch (8) and Warner (16) dismissed cheaply.

Left-arm quick Boult opened the match with a maiden as did medium-pacer Colin de Grandhomme, who almost had Finch out for a duck only for Guptill to drop a tough chance at short extra-cover off a fierce drive.

Boult, however, made the breakthrough in the fifth over when Finch was plumb lbw to a ball that nipped back.

Two balls later Khawaja almost went for a duck, only for a diving Guptill to drop a catch at second slip.

New Zealand fast bowler Lockie Ferguson, however, struck first ball.

He appeared to surprise Warner, who came into this match as the tournament's leading run-scorer with 500 runs, courtesy of a sharply lifting delivery the left-hander could not evade as he tried to sway out of the way before wicket-keeper Tom Latham held a simple catch.

Ferguson also contributed to Steve Smith's downfall, the former captain falling for just five when he miscued a hook shot and Guptill, now at short fine leg, held a sensational diving catch as he atoned for his earlier misses in style.

First-change Ferguson had now taken two wickets for three runs in eight balls.

Warner and Smith were booed on and off the field following their recent returns after each served 12-month bans for their roles in a ball-tampering scandal during a Test in South Africa.

They were both back in the pavilion when Jimmy Neesham took two wickets for no runs in four balls to reduce Australia to 92-5.

Neesham had Marcus Stoinis (21) caught behind before the all-rounder held a brilliant diving left-handed catch off his own bowling after dangerman Glenn Maxwell (one) miscued a pull.