Australia captain Michael Clarke put Saturday's thrilling Cricket World Cup defeat to New Zealand down to his side's inability to defend the swinging ball.

Kiwi quick Trent Boult stepped up with a five-for as Australia were bowled out for 151 in Auckland.

However, the Black Caps made a meal of their run chase and ended up securing the victory by a solitary wicket as Boult clung on alongside Kane Williamson.

Clarke suggested afterwards that playing with a Twenty20 mentality in the 50-over format may have cost his side their unbeaten start.

He told reporters: "We were extremely poor, there is no doubt about that. 

"I think credit needs to go to New Zealand bowlers. They bowled really well. 

"They swung the ball nicely and bowled good areas, but our shot selection was very poor, and I thought our defence more than anything else was an area that was a lot poorer than we would have liked. 

"I think sometimes you can get caught up in Twenty20 cricket and up with day cricket working on the power side of your game. 

"I don't think we've had too many sessions where we've worked on the side of our innings and defending a brand-new ball or a swinging ball, so that is an area that we can certainly focus on moving forward."

Clarke was making his first appearance of the tournament after recovering from hamstring surgery, and was similarly withering in his assessment of his own performance after making just 12 from 18 deliveries.

"I'd like some more runs," he said. 

"I'm no different from the other top six batters. We were disappointing today, and my shot selection was poor."