A coroner will decide if the investigation into the death of Matthew Rosewarne should have been treated as the final act of a man who had hit rock bottom — or a murder.

Key points: Matthew Rosewarne was found dead in a shed at his Palmerston home on May 30, 2016

Matthew Rosewarne was found dead in a shed at his Palmerston home on May 30, 2016 Police treated the death as a suicide

Police treated the death as a suicide A coronial inquest is now examining if it should have been treated as a homicide or a suicide

The 45-year-old was found dead in the shed of his Palmerston home on May 30, 2016 with a gunshot wound to the back of his neck.

Police treated the death as a suicide.

Yet that is now being questioned.

On Tuesday, a Darwin court heard that the handgun found near Mr Rosewarne's body had its safety switch on.

The gun was also said to contain traces of DNA that could have belonged to other people.

The last person to see Mr Rosewarne alive was his former partner Christine Staines, the court was told.

The court also heard that an independent forensic consultant raised concerns about an "investigative bias" during the police investigation.

The NT Police Commander Crime said that although suicide was plausible, the involvement of an unidentified third party in the death "cannot be ruled out", the court heard.

Hitting 'rock bottom'

During the first day of the coronial inquest at the Darwin Local Court on Tuesday, counsel assisting the coroner Kelvin Currie said at the time of his death Mr Rosewarne was in financial trouble, drinking excessively and had recently separated from his long-term partner, Ms Staines.

Two days before the death, he said Mr Rosewarne had been hunting at Gun Point, was seen to be intoxicated and crashed his car as he arrived home.

He said his ex-partner Ms Staines then went to his house and called police later that night saying she had taken two of his guns because she was afraid for Mr Rosewarne's welfare.

On May 30, she rang police requesting they check up on Mr Rosewarne.

They found him dead about 2:30pm.

In the weeks before his death, the court heard Mr Rosewarne had told a hospital clinician he had been in a "low mood" for about six months, but denied suicidal ideation, thoughts of self-harm or harm to others.

He told a mental health team at the hospital he had hit "rock bottom" and needed to sober up.

"He said he dealt with stress by consuming a carton of beer and bottle of vodka each day," Mr Currie told the court.

The mental health team found he was fit to keep his firearms licence, as he was "an active member of the gun club and needed such structures and groups to facilitate his recovery", the court heard.

"Christine [Staines] recalled: 'Guns were his life'," Mr Currie told the court.

Gun had safety switch on, court hears

Senior Constable Gino Rob, who was involved in the crime scene investigation, told the court "the one thing that wasn't consistent with suicide was the safety catch" on the gun next to Mr Rosewarne's body, which was on.

Matthew Rosewarne was found dead on May 30, 2016. ( Supplied: NT deputy coroner )

He said he thought it was improbable that it may be engaged when dropped, but it was still more than likely a suicide.

It was one of the points raised by Mr Currie that he believed should "cause some hesitation".

He also pointed out the bullet entered the back of his neck and exited under his chin, and that Ms Staines was the last person to see him alive.

"There is no independent evidence to indicate he was alive from that point [when he last saw Ms Staines]," he said.

Just last week Mr Currie said police provided evidence that showed the gun, found near the body, had DNA on it that could have belonged to other people.

But despite these factors, Mr Currie said within an hour of finding Mr Rosewarne's body, police believed he had taken his own life.

Therefore his death was investigated as a suicide.

"The story generally is of a man who may have, sadly, had the motivation to take his own life," Mr Currie said.

"However there is no history of suicidal ideation or threats to end his own life.

"There was no suicide note or indication at the time that he may take his life."

This was at odds with claims made by Ms Staines in court on Tuesday — she said there had been times Mr Rosewarne said he "just wanted to die".

She also alleged there were times her former partner had hit her and said she was "extremely" physically fearful of him when they separated.

While Ms Staines said she did not like guns, she said she knew the code to his gun safe and had accessed it to see what was in there.

She also said she had taken photographs of everything in his shed, in case they had to go to court over financial issues.

While she said Mr Rosewarne had a few enemies she "didn't think anyone would take it that far".

Mr Currie said there was no information to suggest the guns taken from Mr Rosewarne's house by Ms Staines two days before his death were ever inspected by police.

'Investigative bias'

Mr Currie told the court there were "some issues with the level of assurance" that the cause of death was suicide and this had been raised with police.

He said police sent their investigative brief to the coroners officer in November 2017, attached with a memo from the detective senior sergeant.

"After considering all the available evidence, the only logical hypothesis is that the death was suicide …," it began.

However, independent forensic consultant Dr Mark Reynolds had reservations.

Mr Currie said Dr Reynolds had highlighted the role of investigative bias in forensic decision-making throughout the investigation.

After reading Dr Reynold's report, and conducting further enquiries, Mr Currie said the Police Commander Crime disagreed with the previous police brief.

"Based on the available evidence, suicide from a self-administered gunshot to the back of the neck is plausible and a possible cause of death," a statement of his, read by Mr Currie, said.

"The involvement however of an unidentified third party in the death cannot be discounted."

Senior Constable Gary Hunt was involved in the investigation of the crime scene and told the court on Tuesday if he had the information he had now he "would probably do a lot of things differently".

The inquest continues.