Dr Thomas Gibson, a specialist in impact injury, told Wollongong District Court that fractures to Mr Neilson's eye socket (orbit) and upper jaw (maxilla) were most likely caused by a strong, single blow to the front of his face.

Prosecutors allege Mr Neilson's cousin, Jason Alfred Cavanough, punched the 60-year-old in the face during an argument outside the Emerald Chinese Restaurant, Woonona, on the evening of January 25, 2012.

Mr Neilson fell to the ground and hit his head on the pavement.

He died in hospital five months later.

Cavanough pleaded not guilty to manslaughter; he is claiming that he only pushed Mr Neilson.

But the court heard Dr Gibson believed the fracture to Mr Neilson's upper jawbone was caused by a single blow to the face.

"There's a fair bit of strength in it [the maxilla]," he said.

"For that to be broken, there would have to be a fair bit of force coming to the front of the face."

When asked if a punch from a fist could constitute such a force, Dr Gibson replied "Yes".

"So more than a mere push?" prosecutor Michael Fox asked.

"Yes; you would know you'd been hit. It's not just a push."

Dr Gibson initially rejected suggestions that the facial fractures could have been caused by Mr Neilson hitting the back of his head on the pavement.

But when pressed during cross-examination, Dr Gibson conceded that he couldn't rule it out as a possibility.

The court also heard during questioning from Cavanough's lawyer that Dr Gibson was not given information related to the height of the two men, their relative weight, and how far apart they stood apart at the time of the altercation.

When asked if this information could have made a difference to his findings, Dr Gibson said it was hard to know.

"More detail doesn't necessarily mean you know more about what happened," he said.

"It's very hard to know whether it would make a difference."

The trial will continue on Monday, with a witness due to give evidence via video link from Japan.