A woman who fatally stabbed her partner at a family gathering has told WA's Supreme Court she made the decision to pick up a knife during a violent scuffle because she felt she "had no other way" of protecting herself and her parents.

Key points: Tracy Bridgewater is on trial for stabbing her partner to death during a fight involving her parents

Tracy Bridgewater is on trial for stabbing her partner to death during a fight involving her parents She has told the court her partner had been violent towards her before

She has told the court her partner had been violent towards her before She said she armed herself with a knife in an act of "self-preservation"

Tracey Elizabeth Diane Bridgewater, 41, was giving evidence in her own defence at her trial on a charge of unlawfully killing Nicholas Josephs at their Hamilton Hill home in September 2018.

The court heard Mr Josephs was stabbed in the chest after a violent argument which included him throwing Ms Bridgewater across a room, throwing her mother by the hair to the floor and strangling and repeatedly punching her father.

Ms Bridgewater's parents had arrived in Perth from the United Kingdom about 10 days earlier to celebrate her 40th birthday and to meet Mr Josephs for the first time.

The court was told the violence started after Mr Josephs refused to turn down the volume of heavy metal music he was playing.

Tracey Bridgewater said her partner had been violent towards her before, and she thought he would kill her. ( Supplied: Facebook )

In her evidence, Ms Bridgewater said she threatened to call the police, but he knocked her phone out of her hand, and pinned her against the wall with his hands around her neck.

She said when her father tried to intervene, Mr Josephs threw her across the room and she lost consciousness.

She told the court when she came to, she saw her father being repeatedly punched.

"I was very, very scared, and I was in shock."

"I couldn't believe what was unfolding before my eyes … I don't like violence and I'd never seen anything like this. I didn't know what to do."

'A self-preservation, automatic decision'

Ms Bridgewater said to try to get Mr Josephs off her father, she decided to scratch him on the face, but he bit her finger.

She said when he released his bite, she ran away, because she did not want him to turn on her, and decided to seek help from neighbours but she could not find anyone.

She testified she returned to the house and saw her mum "in a really bad way" and her father still being punched.

Ms Bridgewater said she then saw a knife on the counter, and had a flashback to an incident two months before in which Mr Josephs had picked up a knife during an argument.

"I thought if he comes past this counter he's going to see this knife and I thought I'd better hold it myself as protection."

"I had no way of protecting myself or my family. I'd exhausted all my other options.

"It was a self-preservation, automatic decision."

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

She described holding the knife by the handle with the blade pointing out, as Mr Josephs yelled he was going to "kill them all".

"He's seen me, he was trying to get to me, my mum was trying to hold him back," Ms Bridgewater said.

"He looked like a crazed man — I thought he was going to kill me."

"I remember Nik towering over me … I had the knife, the next thing I know there was blood on his shirt."

Prosecutors argue her actions were deliberate and not justified or excused by law.

Under cross examination from prosecutor James Mactaggart, Ms Bridgewater denied she deliberately stabbed Mr Josephs.

"I accept I had the knife in my hand ... hence I say I stabbed him, but it was not done with any intention or deliberately," she said.

History of violent abuse

Earlier in her evidence, Ms Bridgewater said there had been other times in their relationship when Mr Josephs had been violent to her, including pinning her to a couch and threatening to strangle her in front of other people in a bar.

She also said in the July incident when Mr Josephs had previously picked up a knife, she had called police and he was given a 72-hour order to leave the house.

Ms Bridgewater told the court as the relationship had progressed, Mr Josephs had become jealous and at times angry, and she had gone with him to a doctor to arrange psychological counselling for him.

The trial continues.