The Australian Under 20 team has lost star fullback Kurtley Beale for the remainder of the IRB Junior World Championship in Japan.

Scans have revealed a tear in one of Beale's hamstrings, sustained midway through the first half of the 38-5 pool victory over Wales in Fukuoka on Saturday night.

The Australians will play New Zealand in a semi-final on Wednesday in Tokyo, with South Africa playing England in the other semi-final the same evening.

"It's disappointing to lose Kurtley," said coach David Nucifora.

"He was contributing very well on and off the field since we arrived."

Beale, from the NSW Waratahs, is flying out of Japan Sunday night and Peter Betham, from the ACT Brumbies, is heading to Tokyo to join the squad.

Betham, 20, played at the IRB Junior Worlds in Wales last year.

At 98kg and 1.92m, he is a powerful runner with good pace.

The Australians advanced to the final four after topping Pool D with victories over Canada (86-0), Tonga (40-6) and Wales (38-5).

The 164 points they scored in the pool rounds was the best of any nation.

Against Wales, the Australians ran in four second half tries to crush the after four penalty goals from halfback Richard Kingi had them leading 12-0 at halftime.

Queensland winger Rod Davies crossed twice during the second half, while second rower Rob Simmons and prop Albert Anae also collected tries.

"Even at halftime we were travelling well," said Nucifora.

"We had to run into the wind in that first half and to go to the break with a 12-point advantage was an indication of how well we combined across the park."

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