The mother of a woman on trial for manslaughter has told the WA Supreme Court she thought her daughter's partner "was going to kill us" during a terrifying argument that ended when he was fatally stabbed.

Key points: Diane and John Bridgewater had travelled from the UK to visit their daughter Tracey

Diane and John Bridgewater had travelled from the UK to visit their daughter Tracey They got into a fight over loud music with her partner Nicholas Josephs

They got into a fight over loud music with her partner Nicholas Josephs This escalated to the point where Tracey Bridgewater stabbed him in the chest

Diane Bridgewater gave evidence at the trial of her 41-year-old daughter, Tracey, who is alleged to have deliberately stabbed her partner, Nicholas Josephs, during a violent argument at their Hamilton Hill home in September 2018.

Mrs Bridgewater and her husband, John, had arrived in Perth from the United Kingdom about 10 days before the stabbing to celebrate their daughter's 40th birthday and to meet Mr Josephs for the first time.

Today, Mrs Bridgewater testified that on the day of Mr Josephs's death, all four of them had gone to a local bar before deciding to return home to finish off food that had been left over from her daughter's birthday party the day before.

Loud music escalated to violence

She said as they were sitting around chatting, Mr Josephs started playing some loud "horrible" music.

She told the court that when her daughter asked him to turn the volume down, he turned it up louder.

Mrs Bridgewater said that led to a scuffle in the hallway involving her husband, her daughter and Mr Josephs, and she could see pushing and shoving taking place.

Tracey Bridgewater is accused of stabbing her partner to death in Hamilton Hill. ( ABC News )

She said she heard her husband say something to Mr Josephs like, "I think you better leave, mate".

"I asked what was going on, what's happening … he [Mr Josephs] said, 'you're a fat slag' and he grabbed my hair and pulled my plait," she said.

She said she was "terrified" of Mr Josephs and feared he was going to kill all three of them.

"I went straight to the ground. Nik was shouting, 'I'm going to kill you, I'm going to kill all of you'."

Mrs Bridgewater testified she then saw Mr Josephs had her husband in a headlock trying to strangle him and was "thumping" him in the face and chest.

"I was terrified he was going to kill John, that John would was going to have a heart attack," she said.

Mrs Bridgewater said she tried to stop Mr Josephs by reaching up between his legs to grab him by "the goolies", but it did not have any effect.

"I was saying to Tracey to get away. She said, 'I can't mum he's biting my finger'," she said.

The next thing she remembered was seeing her husband on the floor and Mr Josephs standing up with a "red dot" on his shirt.

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

She said she told him to sit down but he flung his arms out and swore at her, before stumbling forward and sliding down the wall.

"I sat down next to him and started screaming for help," she said.

"I thought John was dead. I thought he was going to kill us — jump up and kill Tracey and kill us."

Mrs Bridgewater said she then tried to push Mr Josephs and that was when she noticed all the blood.

Multiple injuries sustained in fight

The court has heard Mrs Bridgewater was later treated in hospital for injuries including tenderness to the chest area and a possible fractured rib.

Mr Bridgewater received treatment for a fractured eye socket, a laceration to his left eyebrow and a bite mark to his right forearm.

Tracey Bridgewater has pleaded not guilty to the manslaughter charge.

The court has been told she maintained she acted in defence of herself and her parents when she stabbed Mr Josephs.

The trial continues.