The man charged with murder and attempted murder after he allegedly stabbed his ex-wife and killed her new partner in a Sydney restaurant last night had a restraining order against him.

Alexander Villaluna, 44, allegedly confronted his ex-partner Jovi Pilapil, 38, and her partner Keith Collins, 53, at the Korean restaurant Kangnam BBQ in Hornsby Westfield.

Families and diners ran for their lives when a man entered a restaurant and began stabbing two people having dinner, police said.

Mr Collins, a father from the Central Coast, died at the scene while Ms Pilapil suffered stab wounds to her chest and arms.

Kieran Collins was allegedly stabbed to death at the Hornsby restaurant. (Facebook)

She is in a stable condition at the Royal North Shore Hospital.

Police said Ms Pilapil had taken a restraining order out against her ex-partner just last month.

Mr Collins' daughter took to Facebook today to remember her late father.

"I love you more than anything dad, I'll forever be your girl no matter what," she wrote.

The suspect is arrested by police. (AAP)

Witnesses said she had fled from the scene and was found hiding in a nearby restaurant.

"The guy jumped over the counter and stabbed him with a glass bottle. He got two in the neck and two in the stomach and he like dropped," a witness told 9NEWS.

"It was an argument because the guy cheated on his missus for him. The guy was like 'well, you had my missus'."

Detective Acting Superintendent Damian Henry said the attacker sat, covered in blood, beside a fountain for about five minutes waiting for police to arrive.

Police at the scene in Hornsby. (9NEWS)

Video from the scene shows the accused attacker sitting down in the restaurant as a police officer tells him to stay still.

Witness Michael Short said he saw a man "walking past me covered in blood holding a massive hunting knife".

Fearing for the safety of others in the shopping centre Mr Short grabbed a tyre lever and followed the man.

He claimed Westfield security did not show up until well after police arrived.

"Not saying they had to arrest or stop him, but someone had been stabbed. No first aid kit. Nothing," he said.

It is believed the three people involved knew each other. (9NEWS)

A Westfield spokeswoman said she could not comment on the specific security arrangements but said the security team was working with police.

Police have asked anyone with mobile phone footage of the attack to come forward.

Mr Villaluna has been charged with murder and attempted murder.