Police are baffled by the motive for the killing of 66-year-old Bernd Lehmann who had had been working as a disability carer before he was found dead in a home unit at Alt Street, Ashfield, about 7pm on Valentine's Day 2008.

A post mortem examination revealed he died from severe head injuries.

Strike Force Navala, comprised of detectives from Burwood Police Area Command and the State Crime Command's Homicide Squad, was formed to investigate his death but no arrests have been made.

Following a coronial inquest in 2012, the matter was referred to the Unsolved Homicide Unit.

Police announced a $1 million reward on Thursday and renewed their appeal to the community for any information.

In 2018 a forensic review of DNA samples obtained during the initial crime scene was conducted with samples sent to the Genetic Ancestry Laboratory, managed by the University of Canberra and University of Technology Sydney for DNA phenotyping.

The analysis revealed the DNA samples did not belong to Mr Lehmann, and probably belonged to a man with a multi-racial background including European, Middle Eastern and African.

The DNA phenotyping also indicated the individual most likely did not have blonde hair or blue eyes.

Homicide Squad Commander, Detective Superintendent Danny Doherty, said detectives would explore every line of inquiry in hope of getting justice for Mr Lehmann.

"Strike Force Navala investigators have conducted a thorough review of the all evidence obtained over the past 13 years," he said.

"Detectives are confident that advances in DNA technology will assist in establishing the offender's identity.

"We are hoping that someone may come forward with additional information that will help detectives solve Mr Lehmann's murder."

Australian Associated Press