The women, both aged 20, died after their car was submerged in the Aveley lake early on Sunday morning.
Bystanders tried to rescue the pair, identified by local media as nursing students Nidhi Lalji Hirani and Ruxmi Premji Vaghjiani, but were unable to save them.
Ruxmi's uncle KP Halai told Nine News the close friends had called him asking for help but by the time he arrived, it was too late.
"Nidhi called me and she just said 'Uncle, we are drowning, the car is going down'," he said.
"I told her on the phone, 'try to open the door or try to kick the glass' ... they said the door cannot be opened."
Police believe the Toyota Corolla made a right-hand turn at Gnangara Road and continued across the median strip and into the lake.
Inspector Craig Collins on Sunday said it was unclear what caused the women to veer off the road.
"It's early stages of the investigation," he said.
"We haven't got anything other than we don't believe there's any other vehicle involved."
The women's loved ones gathered near the lake for a candlelight vigil on Sunday evening.
They were believed to have been driving to work at a nearby deli when they crashed.
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