A former Australian soldier charged over a fatal army truck crash has described how he tried to help his dying colleague, praying and wishing he would wake up.

Alexander Gall, 24, wept as he told the jury how he heard Sapper Jordan Penpraze, 22, groaning after the truck flipped while navigating a sweeping bend on the Holsworthy military base in 2012.

"He was really hurt. He was groaning and I sat with him trying to help him," Mr Gall said.

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