They shared an interest in cars, movies and mixed martial arts and their parents lived in the same street.

Friends described them as an "old married couple" because they'd often fight, then "make-up" a few days later.

But their friendship took a fatal turn when Mr Junghans began to suspect Mr Thomas, or his brother Clint Thomas, had "dobbed him in" to police for growing and dealing illegal drugs.

Fearful and depressed about going to jail if convicted over the large-scale hydroponic cannabis set-up police discovered in Mr Junghans' father's Westleigh home, the 30-year-old told friends he suspected one of the Thomas brothers had informed on him.

Mr Junghans had been arrested a month earlier and charged with eight cultivate and supply drug offences and firearms offences. Police had allegedly found 153 cannabis plants with a potential street value of $300,000.

Officers from Strike Force Ipswich, which was investigating illegal drug supply in the Ku-ring-Gai area, allegedly told Mr Junghans they had received "intelligence" on his criminal behaviour.

At an inquest at Glebe Coroner's Court on Tuesday, Magistrate Carmel Forbes was told Mr Junghans came to the "firm belief" either Matthew or Clint, a customs officer, had informed on him.

On the evening of April 25, 2013, Mr Junghans arrived at the Thomas home on Quarter Sessions Road and said, "someone's dobbed on me". He told their mother, Louise Thomas, that "I need to speak to Clint about that".

He then went into Matthew's room and shot him in the head and the chest before turning the gun on himself.

Counsel assisting the Coroner, Emma Sullivan, said the case appeared to be a murder-suicide.

The officer in charge of the case, Detective Senior Constable Stephen Brown, said friends and family described Matthew Thomas and Mr Junghans as "like an old married couple".

In the weeks leading up to the deaths, Mr Junghans was drinking heavily, concerned his family would lose their house under the criminal assets recovery scheme and he faced five years' jail.

In a tragic twist, Detective Senior Constable Brown said neither Matthew nor Clint had informed on Mr Junghans. Rather, he had been under police surveillance for months. Further, there was no evidence that the brothers were involved in the supply of cannabis, he said.

Senior Constable Douglas Polley, who conducted investigations into illegal drug supply in the area said of Matthew Thomas: "I never met him, I never spoke to him. He never came up in our lines of inquiry."

The inquest will also investigate the cleaning of the crime scene by government contractor BVM Clean Scene. The Thomas family claim they found human remains in Matthew's bedroom after it had been serviced by the forensic cleaners.

The inquest resumes in June.