The driver, 29, underwent a roadside breath test and allegedly returned a positive result. His passenger was being interviewed by police.
He was arrested and taken to Castle Hill police station for further breath analysis.
Officers from The Hills Police Area Command established a crime scene, which will be examined by specialist police from the Metropolitan Crash Investigation Unit.
Councillor for the Epping Ward of the City of Parramatta, Donna Davis, expressed her shock on Facebook.
“A horrific, senseless accident in Oatlands this evening. My heart goes out to those involved, their loved ones, friends and first responders.”