The accused, who can't be identified for legal reasons, is charged with the attempted murder of the man, who was living in the former family home at Littlehampton, in the Adelaide Hills, in 2017.

Opening the Supreme Court trial on Monday, prosecutor Lucy Boord said the accused, 32, and his family had been living interstate when the woman met her future partner through her work.

That man moved in with the couple and their two children and continued living with them when they returned to South Australia in 2017.

Jurors heard the woman told her former partner - the accused man - in June of that year that she wanted to separate, and her new relationship had begun by August.

The accused man moved out of the home but visited in October 2017 after spending the weekend with his children.

It is alleged he asked the woman's new partner if he wanted to see his collection of guns, which were kept in a safe in the garage of the home.

Once inside the garage, Ms Boord said the accused took a rifle from the safe and pointed it at the other man's stomach.

"(The man) at that point realised something was very, very wrong," she said.

Ms Boord said the accused then shot the man in the abdomen and told him "this was no accident, that he intended to kill".

"(The shot man) had to escape or fight back," she said.

"He was now in a fight for his life and arguably his partner's as well."

The court heard the injured man pulled himself up against a pile of boxes and grabbed a nearby axe, before striking his attacker's head several times.

Police were called after the woman heard a gunshot from inside the house and found the men and "blood everywhere" in the garage.

Both men were seriously injured, the accused man ultimately left without the use of his arms and legs.

The trial, before Justice Anne Bampton and a jury, is expected to last four weeks.

Australian Associated Press