Son tried to help dad after park stabbing

There was so much blood pumping out of Benjamin Suttie's throat that his son Zac Elder took off his shirt in a desperate bid to stem the flow.

Yet Mr Elder - at the time just 19 - could still feel his father's blood spurt into his hands through the clothing, a court has heard.

Harley David Wegener, 34, has pleaded not guilty to murder in the Brisbane Supreme Court after Mr Suttie's neck was slashed at Woodridge in August 2018.

Mr Suttie, 37, was assisted by his son after his carotid artery was cut in half in a late night altercation between two groups at Prince Park.

"He (Mr Elder) takes his shirt off, puts it on his throat and feels his father's blood spurting into his hands through his shirt with each pulse - that's what he experienced," crown prosecutor Greg Cummings told the jury.

Mr Suttie's life support was turned off three days later after suffering multiple organ failure caused by severe blood loss.

A specialist had never seen a cut as clean as Mr Suttie's slashed artery in their 15 years at Princess Alexandra Hospital, the court heard.

Medical evidence indicated the weapon used had a sharpness almost identical to a surgical scalpel.

The Crown alleges Wegener responded aggressively after being greeted by Mr Suttie as their respective groups passed in the park back in 2018.

"Nothing happened except Ben said g'day and this fellow started to arc up straight away," Mr Cummings said, pointing to Wegener.

Mr Suttie tried to de-escalate the situation but Wegener reached into a satchel and started slashing towards the face and then neck, the Crown alleges.

Mr Elder didn't see a knife but saw Wegener twice swing his hand at his father, Mr Cummings said.

Mr Suttie suffered a cut under his eye as well as his neck with medical evidence indicating they were caused by a slashing or slicing motion, requiring two applications of force, the court heard.

Wegener had earlier told the court that a 30cm knife he carried to "feel safe" had fallen out of his pocket during a fist fight he claimed Mr Suttie started in the park.

Wegener said he picked up the knife and used the same hand to push Mr Suttie to ward him off and regain his feet, with the blade on its side and pointing away.

Mr Cummings said medical evidence contradicted Wegener's testimony.

"The injury (to Mr Suttie) could not have occurred by this accused thrusting the knife out ... as he demonstrated - it can't happen, it's a lie," he said.

Wegener admitted to owning the "sharp fishing knife" and that it was a birthday present from his brother.

It has never been found after Wegener claimed to have dropped it as he fled the park, evidence Mr Cummings described as "rubbish".

"He sheds the knife and runs, the birthday present from his brother, the thing that he needs to feel safe - does he go back and get it (knife)? No," Mr Cummings said.

The trial before Justice Melanie Hindman continues.