An outlaw motorcycle gang member who shot his partner in the head in her home in north-western Victoria last year has been sentenced to nine years in jail.

Brandon Osborn, 37, said he did not know there was a bullet in the chamber when he placed the gun against Karen Belej's head and pulled the trigger.

Ms Belej, 31, died in her home at Cardross, near Mildura, in May 2016 from a single gunshot wound.

She was an active campaigner against family violence and formerly worked for the Mildura Rural City Council, which is now a White Ribbon accredited workplace.

Osborn had previously been charged with murder, but a plea deal saw that reduced to manslaughter and a firearm offence.

Earlier, defence barrister James McQuillan told the Supreme Court, sitting in Mildura, the shooting was a tragic accident and that his client had admitted he should not have been playing with the gun.

Accused thought gun chamber was empty

Osborn had told the court he was unfamiliar with the gun's loading mechanism and was unaware the chamber spun in an anti-clockwise direction.

He said he thought the chamber was empty when he pulled the trigger.

But while the prosecution argued that Osborn consciously and deliberately pulled the trigger on a loaded revolver, Justice Keogh agreed with the defence that Osborn had not intended to kill his partner.

Justice Keogh said that was evident with Osborn's immediate call to triple-0 and his sustained attempts to revive Ms Belej.

But he was critical of Osborn's actions, saying someone with extensive firearm history and training should have been aware of the dangers of handling a gun in such a way.

He said Osborn's actions were profoundly stupid.

"The circumstances of her death are unbearable. It was an appalling act of recklessness," Justice Keogh said.

Justice Keogh said Osborn had taken the life of a much-loved woman.

He said had it not been for Osborn's early guilty plea, as well as his good prospects for rehabilitation and genuine remorse, the sentence would have been longer.

Osborn will be eligible for parole in six years.