Alexander Villaluna, 45, stabbed his former partner Jovi Pilapil, 39, before killing the man with whom she was having dinner, Keith Collins, 53, in the food court of Westfield Hornsby in March 2016.

In the NSW Supreme Court on Friday, Villaluna pleaded guilty to one count of murder and one count of wounding with the intent to cause grievous bodily harm.

Before the attack, Ms Pilapil, an aged-care worker, had separated from Villaluna and had successfully applied to have an AVO in place against him.

She had met Mr Collins, an Irishman and a father who lived in Terrigal on the Central Coast, on an online dating website and they met for their first date at the Kangam BBQ restaurant.

The Herald reported at the time that Mr Villaluna stabbed Ms Pilapil before turning on Mr Collins in front of terrified diners.

Ms Pilapil was able to run away and hide in an upstairs shop, while Mr Collins died at the scene from stab wounds to his neck and throat.

Witnesses said Mr Villaluna then calmly walked up and down the shopping centre's concourse, covered in blood and with a knife in his hand.

After about 15 minutes, a female police officer with her Taser drawn apprehended Mr Villaluna, who had walked back inside the Korean BBQ restaurant and surrendered without incident.

After his death, Mr Collins' daughter, Aisling, posted a photo of him online, writing: "I love you more than anything dad, I'll forever be your girl no matter what."

Mr Villaluna, a Filipino man in Australia on a visa, is due to face a sentencing hearing on September 22.