The Southern Motorway crash in which Shannon and Danielle Kiriau were killed in May last year. Photo from ODT files.

The learner driver of an illegally modified Honda has admitted killing two Mosgiel siblings, as further details of the horror crash emerge.

Cameron Charles Presland (21), of Milton, yesterday pleaded guilty to the manslaughter of Danielle (17) and Shannon (22) Kiriau and two further charges of dangerous driving causing injury to his car's other occupants.

Presland had been drinking before he got behind the wheel and was speeding when his unregistered and unwarranted Honda Integra crashed on the Southern Motorway early on May 18 last year.

A police summary of facts said a sample taken at Dunedin Hospital almost three hours after the crash showed Presland's blood contained 79mg of alcohol per 100ml of blood. The legal limit at the time was 80mg.

Justice Rachel Dunningham, of Christchurch, heard the plea via audio visual link during Presland's appearance in the High Court at Dunedin.

Presland's car was travelling between 142kmh and 163kmh at the time of the crash, the summary said.

He lost control of the vehicle while negotiating a moderate right-hand bend.

The car spun into a tree and the left side of the vehicle struck a metal light pole, killing Mr Kiriau.

Miss Kiriau and another passenger, Courtney Donald, were thrown from the spinning vehicle on to the road.

Miss Kiriau, who was Presland's girlfriend, died at the scene from head injuries.

Miss Donald suffered serious head injuries and fractures and was taken to Dunedin Hospital in a critical condition.

The car's other passenger, Caitlin Adams, suffered fractures to both hands, concussion and bruising.

Presland told police shortly after the crash he believed he was travelling at the speed limit of 100kmh at the time, the summary said.

He admitted to driving the car with only a learner licence and that he was aware the vehicle was unregistered and unwarranted.

None of the car's passengers held driver's licences and Presland was in breach of his learner driver conditions.

When interviewed by police in June, he explained the alcohol in his blood was as a result of ''about three bottles of Woodstock Bourbon and Cola''.

Presland had swapped a van for the Integra just days before the crash.

The Honda had an after-market turbo, which was not certified, the car was unregistered and had not passed a warrant of fitness test since 2010.

Presland was aware of the vehicle's condition when he took possession, the summary said.

The day before the crash he was seen twice ''rapidly accelerating the vehicle in an exhibition of speed''. On both occasions he was carrying passengers.

Presland and the crash's four victims drove to a party in Chapman St later, arriving about 9.20pm.

The group left the party, where Presland consumed alcohol, about 2.45am.

He crashed the Integra shortly after 3am, soon after entering the Southern Motorway.

The road was dry and free of traffic at the time. Weather conditions were clear.

Justice Dunningham remanded Presland for sentencing on June 2.

timothy.brown@odt.co.nz