"The damage to Monis' head was caused by bullets was of such a nature one couldn't see enough there to be able to tell you how many bullets struck it," he said.
"When a bullet hits a head, the temporary wound cavity is massive.
"The fluids in the skull expand. And because it's encased by the skull the skull bursts, basically like a balloon."
"I wouldn't expect fragments to necessarily do that. I would expect a lot of velocity."
The inquest heard that Monis' torso, was struck multiple times, mostly by fragmented bullets.
"Due to the damage caused it's very hard to say," Mr van der Walt said. "His body was not struck by stable bullets in flight.
"They mostly started tumbling or fragmenting by the time the shots or bullet fragments penetrated his torso.
"(Monis) had one bullet wound to his right foot. But the majority of wounds were (in his upper body)."
Mr van der Walt said the gunman cowered or fell and went "limp" behind a chair when he was shot.
The inquest this morning also heard that Monis appeared to have re-loaded his gun after he murdered Mr Johnson, just seconds before police shot him dead.
Mr van der Walt agreed it might "have been the case" that he was in the process of reloading "when the operation ceased".
Mr van der Walt said evidence was consistent with "re-loading taking place" but that it could have just appeared that way because of how Monis fell when he was shot by police.
The inquest yesterday heard a bullet from a police rifle fragmented as it travelled through a chair before hitting Ms Dawson in the rear right shoulder, causing severe injuries.
Ms Dawson was hit in the right shoulder, right upper back, left shoulder, left upper back and neck area, and twice in the right side of the neck.
Some of the wounds contained copper fragments, confirming the were fired by TOU officers' weapons as the ammunition used by Monis didn't contain copper.
The inquest before NSW Coroner Michael Barnes continues.
"The damage was done to his head, and falling and he's either cowering behind the chair or he's limp," he said.
Mr van der Walt said Monis' backpack - which the gunman had earlier falsely warned contained a bomb - was riddled with bullets.
"A large area of the backpack sustained a lot of damage," he said.
