A man charged with murder and attempted murder over a stabbing attack on a couple at a shopping mall on Sydney's upper North Shore may have been motivated by jealousy, police say.

Police said Jovi Pilapil, 38, and Keith Collins, 53, were dining at a restaurant in Hornsby Mall when the woman's former partner, 44-year-old Alexander Villaluna, confronted them before allegedly stabbing them each a number of times just before 9:30pm.

Mr Collins, from the Central Coast, died at the scene. Ms Pilapil ran away and took shelter after she was stabbed in the chest.

The couple had been on their first date at the Kangnam Korean BBQ restaurant.

Acting Superintendent Damian Henry said police thought it was a targeted attack.

"When anybody dies in sudden and unnatural circumstances it's a tragic event," he said.

"The area where the incident occurred is a place that is frequented by members of the public [and] this is certainly out of character for that area.

"Generally speaking it's a very safe area around that part of the shopping mall."

A witness reported seeing the alleged attacker covered in blood and carrying a large hunting-style knife. ( AAP: Mick Short )

One witness, who saw the incident from outside the restaurant, said the man was stabbed several times in a fight.

"He stabbed him with a glass bottle," the witness said.

"He [the victim] like dropped, and it was pretty messed up the way he dropped ... and he was yelling out 'help'."

Another witness reported seeing the alleged attacker covered in blood and carrying a large hunting-style knife.

Sorry, this video has expired Man killed in stabbing at Korean restaurant in Sydney

Superintendent Henry would not confirm what weapon was used, except to say a number of items had been recovered at the scene.

"They're being forensically collected and they'll be examined as part of the investigation," he said.

"A number of people may have some CCTV footage on their mobile phones.

"We're appealing for anyone last night who might have heard something, seen something, filmed something or is in receipt of some of that footage to contact the Hornsby detectives."

The 38-year-old woman was taken to Royal North Shore Hospital with a punctured sternum and was expected to have surgery.

The 44-year-old man was arrested nearby and was taken to Hornsby Police Station where he was charged with murder and attempted murder.

He was refused bail and chose not to appear when his case was heard before Hornsby Local Court.

He will face court again in May.