Strike Force Woolbird was established by Burwood detectives to investigate the circumstances surrounding the women's deaths.
Police are yet to receive the toxicology results or coroner's report, with Allcroft saying the fact that their bodies were "there for some time" made matters "problematic".
"Detectives are interested in speaking with anyone who may have seen or who may have information about the women's movements in the days and weeks before their deaths – which we believe occurred in early May," Allcroft said.
"Extensive inquiries have been made by Strike Force Woolbird detectives; however, we have been unable to determine the exact circumstances surrounding Asra and Amaal's death."
The two sisters arrived in Australia from Saudi Arabia in 2017, with Allcroft saying police "don't know a lot about the girls", and they seemed to "keep to themselves".
"The Burwood community is a small and close community, and we hope that someone may be able to assist our investigators – either through sightings, or those who knew the sisters and may have some information on their movements before their death," she said.
Police have spoken to the family in Saudi Arabia and they are assisting police with their inquiries.
Allcroft said there was "nothing to suggest" that the family were suspects, who have instructed the Saudi consulate to act on their behalf.
The welfare check took place in June, when the landlord contacted the sheriff's office after they had not received rent for more than a month.
Police previously attended the unit in mid-March after a call from the building manager.
"I believe food had been left out in the common areas and he contacted police as he was concerned for their welfare," Allcroft said.
Allcroft said at the time the girls "appeared to be fine" and "no issues" were raised during that visit, with no further action taken by police.
