Two boys in their late teens were sleeping in a sleepout and were not aware of what had happened.
Police would not release the names of the man and woman, but confirmed they and their two teenage sons were the only occupants at the holiday park.
Detective Senior Sergeant Malcolm Inglis, who is heading the investigation, told a press conference in Oamaru this afternoon the teenagers were "pretty shaken and shocked'' and were "unaware of any incident at the house''.
They were receiving support from police, and victim support was helping them.
Both the crash and the woman's death were being investigated as homicides, which was why the vehicle the man was driving and the truck remained at the crash site until late today, reducing the State Highway to one lane and creating delays for Easter traffic.
The truck driver received minor injuries, was treated at the crash site and was now home, receiving help from victim support because of shock.
Det Snr Sgt Inglis said police were investigating both incidents as homicides for the coroner, and their investigation would among other things look at whether the man had deliberately driven at speed across the centre line into the path of the truck, or whether a wet road or other conditions led to the crash.
The crash had occurred at "a reasonable speed'', he said.
It appeared no firearm was used in relation to the woman's death and the circumstances were still being investigated. Asked if a knife was used, he said that was part of the investigation.
About 20 police and a specialist team from Environmental and Research were expected to continue investigations at the holiday park tomorrow.
With no detour available, the highway was closed for about five hours before being opened intermittently for one-way traffic. Later, one lane opened permanently while serious crash investigators investigated.
Police, St John and fire units from Hampden and Oamaru were called to the crash where they were greeted with the badly smashed Freelander and debris spread about 50m across the road and verges.
The Freelander ended up at an angle of about 45 degrees to the road on the east side, while the truck went through a fence into a paddock trying to avoid the collision.
It was damaged on the driver's side and most of the impact on the Freelander was also on the driver's side.
A 43-year-old Timaru man was behind the wheel of the Hilton Haulage truck and trailer unit involved in the fatal collision.
Peter McAuley, company director, said the incident was "a truckie's worst nightmare''.
"The truck driver was on his regular scheduled run, from Timaru [to] Dunedin return. On his return journey, he's had a head on collision with a car.
"It's just a horrendous thing for anyone to have happen to them.''
The man had suffered a minor injury to his hand as a result of the crash,'' Mr McAuley said.
"Staff here, everyone here is just wrapping ourselves around him, looking after him. He's in pretty good shape.''
The truck, which was carrying a load of general goods, had been substantially damaged, he said.
The driver had little recollection of the incident after the airbag inflated, he said.
