Alan William Delaney, 53, is accused of murdering Aaron Baxter by kneeing him in the abdomen numerous times on Boxing Day 2019.

He also punched the deceased in the chest and head, with the latter hitting a concrete pylon on occasions.

Mr Delaney has pleaded not guilty to murder and faced day one of his trial in the Queanbeyan Supreme Court on Monday.

In her opening, Crown prosecutor Nerissa Keay said during the physical altercation under Queens Bridge in Queanbeyan, Mr Delaney accused Mr Baxter of getting involved in his relationship, saying "you told my missus you were taking me to the brothel".

Ms Keay said Mr Delaney also allegedly threatened to drown Mr Baxter in the Queanbeyan River.

The court heard Mr Delaney allegedly continued to make threats as he left the scene at Trinculo Place.

Mr Baxter was taken to Queanbeyan District Hospital but died about two hours later after going into cardiac arrest.

An autopsy showed Mr Baxter died from internal bleeding within his abdominal cavity, originating from laceration to the spleen.

"The laceration from the spleen is associated with the fracturing of the ribs," Ms Keay said.

During a police interview the next day after Mr Delaney called police himself, he said there were other reasons he sought Mr Baxter that morning, including the latter allegedly stealing money from him.

The court heard after police informed him of Mr Baxter's death, Mr Delaney burst into tears and said "please tell me it's not right".

"Please tell me it's a bad dream," he said.

Ms Keay said prior to the incident, Mr Delaney sent his partner a series of text messages and published Facebook posts, including one that read "I've been used for the last time".

Defence lawyer Duncan Berents in his opening said while certain factors, such as the violence and the cause of death, were not disputed, questions surrounded Mr Delaney's state of mind.

Mr Berents said it had to be "a voluntary act of the accused that's caused the death of the deceased" and told the jury to "look at everything else the evidence tells you around the fight".

This included what happened after the accused left the scene, including when Mr Baxter was taken to hospital.

"There is absolutely no dispute that it is a tragic event when someone loses their life," Mr Berents said.

Mr Berents said his client was "upset and angry on Christmas Eve".

"He was upset with the deceased, he upset with his partner. Mind you, he was upset with the world," he said.

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Joseph Wiewiti, who was living in his car under the bridge at the time, said he had known Mr Baxter for about three days prior to the incident.

Mr Wiewiti said Mr Baxter came to greet him on the morning in question.

"He [Mr Delaney] was was yelling, 'I'm gonna f----n kill you, you're a dead c---'," Mr Wiewiti alleged.

"I jumped back in a bit of a fright ... I was pretty much in shock."

Mr Wiewiti said he saw Mr Baxter "trying to walk away and putting his hands up to protect himself".

He said he then saw Mr Delaney allegedly kneeing Mr Baxter in the "ribs and midsection".

The officer in charge of the investigation, Senior Constable Dean James at Queanbeyan Police Station, said when he attended the scene, he saw Mr Baxter on the ground.

"As I got closer, he rolled onto his side and he was groaning. Then he'd roll back onto his back and onto the other side ... again groaning as he did so," he said.

"He was strapped in [to the ambulance], we got to a point where he hadn't moved for some time.

"He looked very, very ill. His colour was very pale. My fear was he had stopped breathing."

The police officer's cross-examination has been reserved as he is set to give evidence again on Thursday.

Mr Delaney has been behind bars on remand ever since he was arrested at his Queanbeyan home on December 27, 2019.

Evidence in the trial, before Justice Desmond Fagan, is expected to take about one week.

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