Reace Hearne, 18, of Sans Souci, appeared before Sutherland Local Court on January 30 when he became eligible for release from custody because of time already served and the non-parole period.

He also received a nine-month good behaviour bond for intentionally damaging the front window of a 7-Eleven service station in the same suburb.

The court heard that an off-duty policeman confronted Hearne at McDonald's after someone said the defendant had a knife.

The officer said he had not produced his police identification when he asked Hearne to drop the knife.

Hearne fled the scene to Caringbah railway station and threw away the 30-centimetre knife.

He told the court that a number of young males had called him names and chased him and he had picked up a knife in the Coles car park before he ran to the McDonald's outlet.

Hearne said he had just wanted to scare the young people pursuing him.

"They were going to bash me," he told the court.

He said he had been drinking alcohol and taken "a white pill" before the incident.

The court also dealt with an incident from July last year after Hearne pleaded guilty to drinking intoxicating liquor on a train and to common assault at Mortdale.

No conviction was recorded for the drinking offence and he was given a 12-month good behaviour bond for the assault.