A man who was allegedly murdered at his trucking business in Adelaide's northern suburbs last year was bashed to death with a two-kilogram ring spanner, a jury has heard.

Key points: The body of David Norris was found at his trucking business in Adelaide's north in September 2017

The body of David Norris was found at his trucking business in Adelaide's north in September 2017 Jesse Ray Penhall was charged with murder and is accused of using a spanner to beat him to death

Jesse Ray Penhall was charged with murder and is accused of using a spanner to beat him to death The Supreme Court has heard from prosecution and defence lawyers on the first day of the trial

Jesse Ray Penhall, 40, has gone on trial in the South Australian Supreme Court charged with murdering David Norris.

The court heard Mr Norris, 39, was found dead in a pool of his blood at his Salisbury trucking business in September 2017.

In his opening address to the jury, Jim Pearce QC said Mr Norris suffered multiple skull fractures.

"At least some of the blows were inflicted as Mr Norris lay on the floor of his workshop… he was probably motionless, he was certainly unable to defend himself," Mr Pearce said.

"His battered and bloodstained body was found later that evening."

The court heard the two men knew each other and had been talking together amicably before the attack.

Mr Pearce told the jury the prosecution was not required to prove a motive, and sometimes murders were simply senseless.

"Whatever the reason for this killing, what is clear is that something went terribly wrong inside that workshop that afternoon," Mr Pearce said.

Mr Pearce told the court witnesses heard the fight and there was CCTV footage from the trucking yard which would be tendered in evidence.

The court heard Mr Norris was bashed with a large ring spanner used for trucks, which measured 60 centimetres in length and weighed two kilograms.

Self-defence a 'critical issue', defence lawyer says

The court was also told Mr Penhall allegedly threw away his bloodstained clothes on the side of a road after the attack, which were later found by police with forensic analysis matching the DNA to the victim.

Defence lawyer Grant Algie QC told the jury a critical issue in the trial would be self-defence.

"If you were satisfied Mr Penhall was responsible for the death of Mr Norris, even if you were to reach that conclusion that would not be the end of the matter," Mr Algie told the jury.

"Because there remains what I would describe as a critical issue in this case for your consideration and that critical issue would be the question of self-defence."

Mr Algie told the jurors they should listen for and consider any evidence of threats made against his client by Mr Norris.

"You might need to consider the nature of any threats that might have been made to Mr Penhall, whether they be verbal threats or physical threats… whether there was fear of perhaps death," he said.

Mr Algie said the jury would need to consider the question of what would be a "genuine and reasonable response to any threats".

Earlier, Mr Pearce told the jury the nature of the attack would prove self-defence was not a lawful excuse in this case.

"Defence is not a cloak for aggression," he said.

"The prosecution will exclude self-defence.

"To strike a man as he lay on the ground is, on the prosecution case, hardly acting in self-defence."

The court heard Mr Norris's movements before his death were known because he was on home detention bail at the time and was being monitored by an electronic device.

The jury will be taken on a view of the trucking yard on Tuesday.