An elderly security guard who reprimanded a drunken man for urinating behind a door at a Bukit Panjang shopping centre was punched to the ground, and died of his injuries almost six weeks later.

Yesterday, 53-year-old Teo Chin Lai was sentenced to three years' jail after being convicted of causing grievous hurt.

He hit Mr Chew Choo Chian, 74, in the upper body along the walkway between Block 440, Fajar Road and Fajar Shopping Centre at about 11.40pm on Nov 18, 2014.

Mr Chew fell backwards and died from blunt force trauma to the cervical spine on Dec 28 the same year.

Teo, who had claimed trial, worked as a food packer at an economical rice stall at a hawker centre in an industrial park.

Deputy Public Prosecutor Tan Si En had sought a sentence of three to 3½ years' jail, saying the key operative sentencing principles in this case were retribution and deterrence.

She said Teo had assaulted an innocent security guard in the course of his duty. Not only did Teo grab Mr Chew's shirt and push him out of the shopping mall, he went on to throw a punch with significant force.

Teo, who was unrepresented, said in mitigation that he had too many drinks that night and did not know that the deceased was working there.

He said he saw the "uncle" fall and wanted to save him.

"I do not wish for this to happen to him," he told the court, asking for a fine. "I would like to apologise and say sorry to the uncle now, and also urge you to be lenient to me because of my age."

District Judge Tan Jen Tse said that due to the seriousness of the offence and the fact the victim had died, a substantial sentence was wholly appropriate.

Teo, whose $30,000 bail has been extended, will start his sentence next Wednesday.

The maximum penalty for causing grievous hurt is 10 years' jail. The offender shall also be liable to a fine or caning.