The driver of an unlawfully turbo charged car that smashed into a lamp post near Dunedin at 180kmh, killing his girlfriend and her brother, had been drinking, police say.
Five young locals were in the Honda Integra when its driver lost control coming around a bend on State Highway 1, at Green Island near Dunedin, about 3am on Sunday.
The car had not passed a warrant of fitness since early 2012 and passengers were not wearing seatbelts, police said.
Three of the five passengers were thrown from the car - two siblings, a female in her late teens and a male in his early 20s, died at the scene.
Three more, including the driver, are in Dunedin Hospital with serious, critical and moderate injuries respectively.
The driver was in a relationship with the young deceased female, a member of the driver's family confirmed to The Press.
The latest tragedy marks the third fatal car crash caused by alcohol and no seatbelts in just over three weeks in Southern Otago, police said.
It brings the total number of deaths in the region to six in the last month.
Police said the driver was travelling at 180kmh before the car spun, slammed into a lamp post and then hit a tree.
Sergeant Craig Brown said the vehicle was mangled.
''It was pretty horrific, the car is a complete write off and to a point it's amazing that anyone survived at all.''
Alcohol was involved, ''speed was a factor'' and the car had not passed a warrant of fitness since early 2012, he said.
The Honda's modified turbo was uncertified and records show the vehicle may have been previously ordered off the road.
The ''condition'' of the vehicle, the speed it was travelling at, the lack of seatbelts and the level of alcohol in the driver's system would all be considered in the police investigation, Brown said.
Police were yet to speak to the surviving occupants of the car as they were still recovering from their injuries.
The crash closed down the Southern Motorway until about midday.
''The message that comes across here is that far too often there are horrific and tragic consequences as a result of speed and alcohol,'' Brown said.
He cited two fatal crashes, where alcohol and a lack of seatbelts were to blame, in Southern Otago in the past three weeks.
Three men were killed when their Toyota Hilux crashed into the edge of a concrete bridge near Waikaia on the early hours of Anzac Day.
Christopher Martin Simpson, 39, Matthew James Kennedy McKee, 39, and Ewan Charles Christie, 59, all died at the scene, north of Gore.
Two days later, in the early hours of April 27, a 19-year-old man died after the vehicle he was in crashed, rolled and came to a stop upside down, near Dunedin.
The young man was one of five occupants, all in their late teens and early 20s.
The other four escaped without significant injury.Brown confirmed alcohol and a lack of seatbelts was a factor in both crashes.
A family member of the driver told The Press that her family did not know much about the circumstances surrounding yesterday's crash.
There was an outpouring of grief from friends and family yesterday afternoon as stylised pictures and tributes made the rounds on social media.
A picture with a portrait of the siblings captioned ''Rest in paradise'' was shared.
People commented saying they were ''Two beautiful young souls. Taken way way to early'', and ''they will forever be in our hearts''.
Friends of the driver urged him to ''stay strong''.
The driver had been convicted previously for driving related-offences.
Today marks the start of Road Safety Week.