The police summary of facts said Siale was partying with Webb-Hirst and Mauheni at a central Oamaru address on the evening of June 12.
Mauheni was not drinking, but Webb-Hirst and Siale were. Siale was later found to have a blood-alcohol level of 59mg. The legal limit for a driver under the age of 20 is zero.
The trio decided to leave in Mauheni's 1995 Nissan Maxima sometime between 10.15pm and 10.30pm.
Where they went and who was driving was not established by police in the aftermath of the crash.
Siale has never held a New Zealand driver's licence.
He was behind the wheel of the Maxima about 11pm as it headed north at an unknown speed on Shortland Rd, a straight, roughly 2.5km-long stretch of road.
As he approached the T-intersection with TY Duncan Rd, Siale dipped the headlights but drove straight through the intersection.
The Maxima slammed into a pile of boulders at the front of a house with such force that the car ended up on top of them.
The impact of the crash rotated and shunted the car's engine towards the driver's footwell, causing a fire in the engine bay. The fire quickly spread.
The occupants of the property rushed to help but were unable to pull the two teenagers from the burning vehicle. Siale was able to free himself.
Mauheni and Webb-Hirst suffered severe head and brain injuries and died at the scene a short time later. Siale suffered serious leg injuries and was in Dunedin Hospital until July 1.
According to the police summary of facts, Siale was familiar with the stretch of road, as he worked on a market garden near the scene.
He initially told police there were four people in the car and that he was a front-seat passenger, but changed his story and admitted there were in fact only three and he was driving.
Siale, who declined to be interviewed by police, was convicted by Judge Joanna Maze and will be sentenced on April 22.
