Sisters Asra Abdullah Alsehli, 24, and Amaal Abdullah Alsehli, 23, were found dead on 7 June in separate beds at home in the suburb of Canterbury.
Police, who were called to the property for a welfare check, said the women are believed to have died in early May.
But despite "extensive inquiries", they still do not know how or why.
The sisters moved to Australia from Saudi Arabia in 2017 and may have sought asylum, The Sydney Morning Herald reported. Police refused to confirm this, saying they do not comment on residential status.
A human rights organisation said it should be established whether the women fled Saudi Arabia because of domestic violence or harsh laws governing women. However, there is no evidence this is the case.
Police said they had been in contact with the women's family, which is assisting them with inquiries.
Lina al-Hathloul, head of monitoring and communications at Saudi human rights organisation ALQST, said it "would not be the first case" of Saudi women who were killed abroad after fleeing domestic violence.
"There are no protections for women who are victims of domestic violence in Saudi Arabia, so they flee abroad," she told the BBC.
She added: "I'm not saying that is the case here, just that we need a thorough investigation. It is frustrating not to have any information."
According to the Sydney Morning Herald, there had been signs that something was wrong.
Last year, the women told their building manager they thought someone was tampering with their food deliveries, the paper reported.
A plumber who visited the apartment also said he believed there was "something mysterious" going on, and that police had been called in the past over concerns for the women.
New South Wales Police issued a renewed plea to the public on Wednesday, saying "any piece of information" could be the key to solving this case.
The local community is close-knit, police said in a statement, asking anyone who may have known or seen the women to come forward.
A report from Australian current affairs programme Four Corners in 2019 found 80 Saudi women had tried to seek asylum in Australia in recent years. Many of them were fleeing male guardianship laws.