The motorist suspected of causing the horrific South Coast crash that killed himself and a Sydney couple, and left actress Jessica Falkholt and her sister fighting for their lives in hospital, was driving home from a methadone clinic.
Crash investigators are looking into why the Toyota Prado being driven by Ulladulla man Craig Anthony Whitall, 51, veered onto the wrong side of the Princes Highway on Boxing Day and collided with a car carrying the Falkholt family from Ryde.
Mr Whitall was killed instantly, as was Lars Falkholt, 69 and his wife Vivian, 60.
The Falkholts' two adult daughters, Home and Away actress Jessica, 28, and Annabelle, 21, were pulled from the wreck in critical conditions just moments before both vehicles burst into flames.
The sisters continued fighting for their lives on Thursday, with Jessica in St George Hospital and Annabelle at Liverpool Hospital. They are being surrounded by family and close friends.
The crash has become the headline of a shocking festive season on the state's roads, with 21 people killed since December 15. A 77-year-old woman was killed on Thursday when her vehicle was involved in a crash with a truck travelling in the same direction on the Hume Highway at Gunning.
But the factors that caused the carnage south of Sussex Inlet look set to spark debate about drivers under the influence of drugs which are not classed as illicit.
The Falkholts were travelling back to Sydney after a short Christmas break when fate carried them close to Mr Whitall.
Fairfax Media has learnt Mr Whitall, a man known to police, was making the hour-long drive home to Ulladulla after visiting a Nowra methadone clinic on Boxing Day morning.
About 50 kilometres into the 65-kilometre trip, as he neared the Bendalong turnoff, Mr Whitall's four-wheel-drive failed to negotiate a sweeping left hand bend before careering into the Falkholts' Mazda.
Methadone is an opioid and is used as a prescription drug to curb withdrawal symptoms for heroin addicts.
It is not illegal for a motorist to drive with methadone in their system, however drivers can be charged if it is proven that any substance impaired their ability to perform behind the wheel.
It is unclear whether an autopsy would be able to determine what drugs may have been in Mr Whitall's system because of the severe injuries he received in the crash and subsequent fire.
The carnage has prompted the federal opposition to call for a national review to determine why the nation's road toll is again on the rise.
"The government needs to have an urgent reassessment of why it is that more people are dying on our roads with tragic consequences for their families and for their communities," Labor's transport spokesman Anthony Albanese said on Thursday.
"This needs an explanation and it needs a strategy of how we can return to a declining road toll."