Sydney father David Campbell, 26, believed the people inside a Nissan Skyline had given him a "death stare".

His friend Scott Wilson tried to calm him down and said, "Let's just go."

Mr Campbell decided to do a U-turn and followed the Skyline back into the Macquarie Park car park, along the Hawkesbury River.

The men in the Skyline said that Mr Campbell yelled out, "What the f--- are you looking at?"

Shortly afterwards, a fight broke out between Mr Campbell and the man who would later kill him - Stephen McGuiness.

A witness reported seeing Mr Campbell pushing at McGuiness' chest and saying, "Come on, fight me," before the pair exchanged several blows.

While Mr Campbell was throwing punches, McGuiness pulled out a knife and stabbed him in the neck.

Mr Campbell stumbled backwards and said, "He stabbed me."

He died at the scene.

On Monday, McGuiness pleaded guilty in the NSW Supreme Court in Sydney to one count of manslaughter.

Outside court, Mr Campbell's sister Patricia Campbell said she hoped her brother's death would prevent violence in the future.

"David was a loving, caring, person - we are a very, very close family and David was very family orientated - David was our rock.

"Our family is now broken and we are all struggling," she said.

There should be tougher punishments for people who carry illegal weapons in public, she said.

McGuiness will face court for his sentencing hearing on December 9.