Melbourne man Hung An Vu says he didn't mean to kill his de facto partner's secret new husband, but wanted to scare his rival when released from prison to learn about the marriage.

The 46-year-old is standing trial for the murder of Hung Viet Nguyen, 44, at Sunshine West in August last year and admits the fatal stabbing but insists it was not intentional.

Vu appeared in the Supreme Court of Victoria on Tuesday, charged with murdering Mr Nguyen and the attempted murder of his partner Laura Chan after he allegedly stabbed her as well.

In his opening address to the jury, crown prosecutor Kevin Doyle said that upon his return from prison, Vu heard his partner was having an affair with Mr Nguyen "and he was angry about it".

He said Vu and Ms Chan had been together since 2001 and had two children.

Then while he was in jail, Ms Chan formed a relationship and married in secret to Mr Nguyen, who then moved into her home.

When Vu left prison, he moved back in and Mr Nguyen moved out.

Vu questioned his sons about the relationship, with one of them finally confessing his mother had been with Mr Nguyen.

On August 5 last year, it is alleged Vu asked Mr Nguyen to meet, planning a confrontation with him and Ms Chan.

"He asked them how long they'd been together," Mr Doyle said.

"Mr Nguyen tried to tell the accused: 'It's not what you think'.

"The accused man became angry."

Vu allegedly produced a hunting knife and stabbed both of them in the chest.

It is also claimed Vu tried to harm himself with another knife.

Ms Chan was taken to the Royal Melbourne Hospital with what the prosecution claims were life-threatening injuries.

Mr Nguyen died at the scene from internal bleeding with the knife having penetrated his aorta.

Vu allegedly first told police Mr Nguyen "did suicide" and that he and Ms Chan had stabbed themselves out of shame for their affair.

He later said he pulled the knife out to "scare" the pair, and that the stabbings were "accidents in the course of a struggle".

Defence barrister Stewart Bayles argued Vu did not intend to kill Mr Nguyen or attempt to kill Ms Chan.

"He never intended to deliberately stab either of these two people," he said.

Vu has also been charged with intentionally and recklessly causing injury to Ms Chan.

The trial continues.

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