Biannca Edmunds, 34, has been charged with murder of Michael Caposiena, shot by Edmunds' husband Glen Cassidy who also ended up dead in their confrontation.

Prosecutors allege Edmunds assisted, encouraged or directed Cassidy to kill Mr Caposiena at his home in Melbourne's Westmeadows in 2016.

Edmunds has pleaded not guilty to the murder charge, with her trial starting before a jury in Victoria's Supreme Court on Tuesday.

The court heard prosecutors must prove that Cassidy murdered Mr Caposiena and that Edmunds assisted or directed him to commit the murder.

Crown prosecutor Neill Hutton told the court that on March 12 2016, Cassidy knocked on the door of Mr Caposiena's home and his girlfriend answered.

Mr Caposiena went to the kitchen, armed himself with a knife and answered the door as Cassidy produced a firearm.

"There was a struggle, there was a shooting, Michael Caposiena was shot in the head through the ear," Mr Hutton told the court.

"During that struggle he managed to stab Glen Cassidy two or three times."

Mr Caposiena died almost immediately.

Cassidy tried to shoot Mr Caposiena's girlfriend but the gun did not go off. He beat her with the weapon before collapsing and dying from his stab wounds.

Mr Hutton claimed Edmunds had helped Cassidy to plan the murder and travelled to Melbourne with him to stake out the Westmeadows property.

He said she helped Cassidy procure the gun and helped him draw a map of the surrounding area, including pointing out security lights and barking dogs.

Mr Caposiena had sought supervised visitation to a child.

Edmunds' sister said she would go on rants about wanting Mr Caposiena out of the child's life, saying "I want him gone", the court was told.

Mr Hutton said Edmunds had told her mother she wished her ex-partner would "piss off" and fall off the face of the earth.

Additionally, Mr Hutton said Edmunds had claimed more than $300,000 from her ex-partner's estate, which would also form part of her motive to have him killed.

Defence barrister John Kelly denied the prosecution's allegations and said Edmunds disputed that she assisted, encouraged or directed her husband.

He said the court would hear from witnesses who were motivated to fabricate and invent their accounts of what had happened.

Mr Kelly urged the jury to consider whether the altercation amounted to murder, given Cassidy was stabbed several times in the lead-up.

Even if it did, he said it was a "long road" to conclude murder was committed at Edmunds' behest.

The trial continues on Wednesday, when Mr Caposiena's girlfriend will give evidence.

Australian Associated Press