Specialist police investigators from Sydney have been sent to investigate after the car crashed into a pole on Tuesday night, catapulted down an embankment into a water channel and flipped onto its roof at Yanco, about seven kilometres south of Leeton.
By the time emergency services arrived around 11.15pm, everyone in the car was dead.
"Whilst yet to be formally identified the occupants are believed to be a 36-year-old woman and three boys aged 10, 11 and 12 who are all related to one another," NSW Police said in a statement on Wednesday.
Leeton mayor Tony Reneker says people in the town have been left shocked by the tragedy which happened just days before the Easter holiday weekend.
"It's a shocking feeling, a feeling of despair and disbelief," he told AAP.
"Every fatal accident is a tragedy but to have four killed in one vehicle is just unbelievable," he said.
"I've spoken to the police commanders ... they've actually got the crash investigation team from Sydney coming down to take over the investigation," he said.
Meanwhile, NSW Police are launching Operation Easter – a highly visible traffic operation focused on reducing road fatalities and trauma this holiday weekend.
The five-day operation will begin at midnight and run until 11:59pm on Monday, with double demerits in place for drivers caught speeding, drink driving, not wearing seatbelts or helmets or using a mobile phone while behind the wheel.
Traffic and Highway Patrol Commander, Acting Assistant Commissioner Tracy Chapman, said substantial police resources had been assigned to the operation.
"The message is simple; drive to the speed limit, focus on the road – not your phone, and make sure you have a plan B if you are drinking," she said on Wednesday.
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