The combination of drugs, alcohol and fatigue led to Lisa Crowley driving off the road and crashing into a tree, killing herself and four others. It was two days before they were found.
Miss Crowley, 23, partner Richard Melling, 37, and their three children, Jordan Melling, 11, Brody Crowley, 2,and a 3-week-old baby named Dylan - were found in wreckage off SH32 near Whakamaru on December 18.
Coroner Gordon Matenga's findings into the deaths and recorded them all as accidental, but said a combination of alcohol, cannabis and diazepam proved to be fatal when fatigue was added to the mix.
Mr Matenga said the family had held a barbecue at their home in Whakamaru on Saturday, December 15. In the early hours of Sunday, December 16 a neighbour heard car doors slamming and a car leave from their address. Just after 2am they arrived at a friend's house in Tokoroa where they thought there was a party.
They stayed for a few minutes and left. At this point Mr Melling was driving their Honda Integra but at some point they swapped and Miss Crowley took over.
"She has come to the end of a long straight section of road and has failed to follow the roadway which bends to the left and instead, has continued to travel in a straight direction onto the incorrect side of the road and then off the road into an area of large poplar trees," Mr Matenga said in his report.
"The evidence indicates the car left the road under power."
He said the car had been travelling at or near 100km/h and as it left the road became airborne and hit the first of three trees.
"The force of the impact has caused the Honda to tear apart through the midsection from left to right and fold around the tree. The impact forces have cause the engine along with the gearbox and windscreen to be projected forward 4.5m and 8.5m respectively."
All five occupants died as a result of injuries from the crash. Miss Crowley had a therapeutic level of diazepam in her blood, alcohol was below the legal driving limit and the presence of cannabis was indicated.
"Combine those three drugs with the lateness of the hour and you have a potentially lethal combination," Mr Matenga said.
"This tragedy highlights the need for drivers to be aware of the need to have adequate rest and to not drive while affected by drugs or alcohol. These deaths were preventable."
