"The SA Police Service (SAPS) and metro police across the province must establish an elite unit to investigate accident scenes and report on all probable causes," said Democratic Alliance Gauteng road and transport spokesman Neil Campbell.

A memorial service was held on Wednesday for 19 people who died when a Putco bus veered off a bridge on the R59 in Meyerton, south of Johannesburg on June 25.

"The inadequate mechanisms for investigating tragic bus accidents in Gauteng are an injustice to victims and their relatives," said Campbell.

A "lack of proper road accident forensics" added insult to the injury of the people left behind, because they did not get proper answers for the deaths, or waited months for an explanation, he said.

This also undermined transport operators and law enforcement agencies' efforts to curb fatal accidents.

Campbell said he would ask roads and transport MEC Ismail Vadi for an independent inquiry into testing centre operations and the issuing of roadworthiness certificates within the province.

Putco said the bus had passed a roadworthy test, but this was disputed by the Sedibeng municipality.

Transport Minister Ben Martin called for an investigation into the cause of the accident.

The Road Traffic Management Corporation had sent accident investigators to the scene.