Police were conducting random patrols in Penrith on Wednesday night when they saw the car in a car park on Tench Avenue beside the Nepean River just before midnight.
On inspecting the vehicle, police found the man, 24, and woman, 34, passed out in the front seats and the girl, who was awake, in the back seat.
Police are unsure how long she had been there.
They called ambulance paramedics who rushed the toddler to Nepean Hospital.
After locating and seizing synthetic cannabis and drug paraphernalia, police arrested the pair.
They were taken to Penrith police station where they were charged with child neglect and possessing a prohibited drug.
The pair were granted bail and will face Penrith Local Court in October on drug and child neglect charges.
The girl is now in the care of Family and Community and Services.
The relationship of the adults to each other and to the child are unclear.
The case sounds eerily similar to an incident in the US earlier this month in which pictures taken by a small-town Ohio police department showed a couple allegedly overdosed on heroin in their car with the woman’s son in the back.
The images, which quickly went viral, were posted on the Facebook page of the East Liverpool, Ohio Police Department, along with an officer’s report of the incident that led to the arrest of James Acord and Rhonda Pasek on child endangerment charges.
“We are well aware that some may be offended by these images and for that we are truly sorry, but it is time that the non drug using public sees what we are now dealing with on a daily basis,” the police department said in a statement on the Facebook page.
“The poison known as heroin has taken a strong grip on many communities not just ours, the difference is we are willing to fight this problem until it’s gone and if that means we offend a few people along the way we are prepared to deal with that,” the department said.
The photos show Mr Acord, 50, and Ms Pasek, 47, passed out in the front seat of their car, their mouths agape, as the child stares silently into the camera from the back.
Officer Kevin Thompson wrote that Mr Acord, whose head was “bobbing back and forth”, his speech almost unintelligible, told him he was taking Ms Pasek to hospital and tried to drive away before falling completely unconscious.
Ms Pasek was also unconscious and turning blue, the officer wrote.
Both were revived by emergency medical technicians using several rounds of a drug used to reverse an opiate overdose, according to Officer Thompson’s report.