A "professional, educated" 41-year-old woman fatally stabbed her partner in front of her parents — who had come to Perth to meet him — after he "exploded" in "an extreme and inexplicable psychotic rage", the WA Supreme Court has been told.

Key points: Tracey Bridgewater's UK-based parents were meeting her partner of two years

Tracey Bridgewater's UK-based parents were meeting her partner of two years They got into an argument with Nicholas Josephs about loud music he was playing

They got into an argument with Nicholas Josephs about loud music he was playing Mr Josephs allegedly attacked her parents and Ms Bridgewater fatally stabbed him

Tracey Elizabeth Diane Bridgewater, 41, is on trial accused of unlawfully killing Nicholas Josephs, 44, at their Hamilton Hill home in Perth's southern suburbs in September 2018.

The court has been told the couple had been in a relationship for about two years. Her parents had come to Perth from the United Kingdom in September 2018 to celebrate Ms Bridgewater's birthday and to meet Mr Josephs for the first time.

On Sunday September 23, 2018, the day after Ms Bridgewater's birthday party, the four of them went to a local bar before returning home, where an argument developed about Mr Joseph playing music too loud.

State prosecutor James Mactaggart told the court a violent scuffle broke out in which Ms Bridgewater's father had his eye socket fractured and her mother was pulled to the ground and possibly kicked.

Mr Mactaggart said it was alleged Ms Bridgewater then "massively escalated what was taking place".

"The accused went to the kitchen sink and retrieved a large black-handled knife … with a very sharp blade," Mr MacTaggart said.

"The accused proceeded to fatally stab Mr Josephs … she deliberately inflicted a wound that penetrated his chest … causing his death."

Father 'punched repeatedly until unconscious'

In his opening statement to the court, Ms Bridgewater's barrister, Jonathan Davies, said his client maintained she acted lawfully in defence of herself and her parents.

Mr Davies said Ms Bridgewater and her parents were that night "confronted with a level of extreme, inexplicable, psychotic rage, the likes of which they had never experienced".

He said after being asked to turn down the music, Mr Josephs "went out of his tree and exploded into a rage".

Nicholas Josephs was stabbed to death by his girlfriend who argued she was acting in self defence. ( Supplied )

Mr Davies said Ms Bridgewater was lifted off the ground and slammed into a wall by Mr Josephs, who then "turned his anger" to her father, pinning him down and repeatedly punching and headbutting him until he lost consciousness.

Mr Davies said Mr Bridgewater had a bad heart and both his client and her mother were concerned he was going to have a heart attack.

But he said when Ms Bridgewater's mother tried to intervene, she was grabbed by the hair and "violently" thrown to the ground.

He also claimed Mr Josephs was yelling threats such as, "I am going to kill you all and kill the dogs".

Mr Davies said when Ms Bridgewater believed Mr Josephs was not going to stop, she picked a knife up from the bench and used it as a weapon and to try to deter him from further attacks.

"She held the knife in front her [and] Mr Josephs lunged towards her," Mr Davies said.

"He looked like a crazy man, she feared for her life.

"She took steps forward, at the same time as he lunged and the blade penetrated his chest."

Mr Davies said the stabbing was the culmination of a "troubled relationship" in which Mr Josephs had been "periodically and irrationally violent" to Ms Bridgewater.

He claimed at times Mr Josephs was "irrationally jealous, controlling and obsessive" towards his client, and she had the year before gone to a doctor to seek help for his "anger and jealousy issues".

The court was told in 2017 she had called police after one of his "outbursts" and he was ordered to leave their home for 72 hours, but he returned and she had to call police again.

'It was self defence', operator told

The court earlier heard Ms Bridgewater immediately called triple zero after the stabbing and when first asked what happened she replied, "I've hit [him] with a knife".

She was asked where and said, "I'm not sure, he's bleeding a lot". But when she was asked again, she said. "In his chest".

Tracey Bridgewater claims she acted in self defence. ( ABC News )

Mr Mactaggart said when police and ambulance officers arrived Ms Bridgewater told them, "I stabbed him" and when advised she was under arrest she added, "but it was self defence".

Mr MacTaggart said it was the state's case Ms Bridgewater had no lawful justification nor excuse for stabbing Mr Josephs.

He said she had taken, "the drastic and unnecessary step of going to the kitchen and retrieving a large knife".

"Her actions drastically escalated an already tense situation … to a new and deadly level."

The trial continues.