Australia Under 20s have survived a second half fightback from France to win 42-21 and secure fifth place at the inaugural IRB Junior World Championship at Cardiff Arms Park in Wales overnight.

The first half performance from Australia was effectively enough for them to secure the win as they played with a stiff breeze and raced into a 30-point lead at the break.

Flyhalf Quade Cooper kicked a first minute penalty and then a slick backline move brought the first try as centre Rob Horne put his lightning-quick wing Ratu Nasiganiyavi into space for a simple run in.

Cooper kicked the conversion and when winger Junior Sovala picked up a loose ball and dived under the posts for his fifth try of the tournament, the Aussies led 17-0 after 10 minutes.

The Australians were completely in the ascendancy as they hassled France off their own scrums and turned defence into attack deep in their opponent's territory.

Another Cooper penalty gave them a 20-0 lead after 23 minutes as the margin looked unassailable for France.

A tough task seemingly became mission impossible when Australia made the most of a turnover and Nasiganiyavi out-sprinted the defence for his second try before the half hour.

The giant wing then completed his hat-trick before the break when his pace took him over again.

France did their best to come out firing in the second half and their efforts paid off when flanker Louis Madaule split the defence for a 49th-minute try as they finally managed to string some good phases together.

It got better for France when their other flanker Arthur Chollon scored another try following excellent driving forward play and fly half Mathieu Belie kicked the conversion to reduce the deficit to 30-14 as the hour mark approached.

Australia steadied the ship with a try from prop Ben Daley in the 69th minute, but France hit back with another try through their powerful centre Mathieu Bastareaud. Belie converted to make it 35-21, but that was as close as they came.

Cooper helped Australia finish the game on a high as he a made a number of superb breaks and Man of the Match Nasiganiyavi set up second row Rob Simmons for the final try of the game before the final whistle.

Ford Australian Under 20s Coach Brian Melrose said it was a good end to the tournament against strong opposition.

"It was nice to finish off in that fashion," said Melrose.

"France are a top nation and both of us would be like to be playing later tonight but we proved we are one of the better sides.

"There was a massive wind but we got some good tries and we hung on the second half and pushed away at the end."

The squad will arrive home in Sydney on Wednesday.