Allan Geoffrey O’Connor is charged with the murder of Rebecca Webb, Steven Cumberland and his son Jacob Cumberland at a remote property at Hermidale in June 2015.

On Monday the trial in the Supreme Court of NSW opened with Justice Robert Hulme presiding.

Mr O’Connor, 64, has pleaded not guilty to each murder charge.

He sat in the dock without handcuffs and wearing trousers, shirt and a tie as the jury was empanelled.

Justice Hulme told the jury - comprising six men and six women - the trial was estimated to run for six weeks.

The Crown foreshadowed 80 to 90 witnesses in the case.

The investigation into the deaths had started after two men went to the property owned by Steven Cumberland, 59, about 8.30am on June 26, 2015 to muster goats, the Crown said.

On arrival they saw one caravan was badly burnt, and Jacob Cumberland, 28, dead on the ground and they called triple zero, the court heard.

The men also saw the burnt body of Steven Cumberland inside the caravan, the Crown said.

The body of third person was lying in 25 cm of water in a pool - this was 36-year-old Ms Webb, the court heard.

The Crown’s case is the accused Mr O’Connor shot each of the victims twice with a 12-gauge shotgun.

The evidence the Crown said pointed to the accused was firstly text messages that in effect Steven Cumberland had taken the accused’s girlfriend and that Mr O’Connor was “very upset”.

Secondly he had “opportunity” - the property was not easy to locate, and Mr O’Connor had been there many times, the Crown said.

The accused was also familiar with firearms and had a shotgun, the court heard.

Thirdly, there were multiple items of forensic evidence, one of which was a pair of boots worn by Mr O’Connor, that after analysis showed human blood that matched the DNA profile of Jacob Cumberland.

Defence barrister Ian Nash said it was not in dispute that Ms Webb, Steven and Jacob Cumberland were killed sometime between the night of June 25, 2015 and the morning of June 26, 2015.

Nor was it in dispute that whoever or whomever killed the three did so meaning they would die, he said.

But what was “very much in dispute” was that the accused had killed them or had something to do with their deaths, he said.

The defence did not dispute that Mr O’Connor had a relationship with Ms Webb in the weeks prior June 26.

With the evidence of the boots, what was in dispute was how blood got there and when, Mr Nash said.

He said he expected the court would hear more about the type of boots Mr O’Connor was wearing, where they came from and “other possibilities” for how the blood got there.

The barrister spoke of whether the jury might hear others had motives to plan the crimes.

The trial continues.