Constable Antonio Saffioti, 51, pleaded not guilty to causing grievous bodily harm and assault occasioning actual bodily harm.

His alleged victim, Leonard Bartholomuesz, 73, suffered a broken ball joint at the top of his leg and a dislocated shoulder after the fall. Six days later he died in hospital.

The case before the Downing Centre District Court on Tuesday centred on what happened during two or three seconds on 30 April 2013 on the lower ground floor of the Woolworths supermarket near Town Hall railway station.

The court heard Mr Saffioti was buying food on his lunch break when he and another two police officers crossed paths with Mr Bartholomuesz, a regular customer, in an aisle.

Crown prosecutor Paul Lynch said Mr Bartholomuesz bumped into Mr Saffioti, who then pushed him to the ground.

The Crown's case relies on the trial's only eyewitness, fellow customer Penelope Randall, who allegedly confronted Mr Saffioti after witnessing the push.

Mr Lynch said Ms Randall asked Mr Saffioti: "Why did you do it?"

Mr Lynch also said the policeman allegedly replied with words to the effect: "If you were shoved at and touched all day like we are, you would do the same."

The only CCTV footage to capture the event was "poor" according to the Crown and Mr Saffioti's defence barrister, Ray Hood, agreed it had limitations. At one point the court's TV screen showing the footage was wheeled close up to the jury box.

But Mr Hood said the jury would not find the Crown's witness credible after viewing discrepancies between her testimony and the footage.

Mr Hood said his client was not using excessive force when he pushed Mr Bartholomuesz.

The trial continues before Judge Christopher Robison.