The driver, a Cromwell man in his 40s, was taken to Oamaru Hospital by Twizel ambulance with a head laceration and bruising.
Mainfreight Cromwell branch manager Justin Marshall said yesterday afternoon the driver was "OK" and the medical checks were precautionary.
The driver's name was not released by police yesterday.
Mr Marshall said the man had about 30 years' experience as a linehaul driver.
He was driving a 44-tonne Volvo FH460 vehicle, which is a combination truck and trailer unit.
The truck was carting a 30-35 tonne load of chemicals from Christchurch to Cromwell.
The accident happened at Dip Creek, about 10km south of the summit, early yesterday.
According to Omarama Deputy Chief Fire Officer Steve Reeve, the truck's manifest was a wide range of chemicals including paints, batteries, solvents, adhesives, weed sprays and other materials.
Mr Marshall said he did not know what time the accident happened and because there was no cellphone coverage, the driver could not raise the alarm.
A passer-by raised the alarm about 2.30am, Mr Marshall said.
The Omarama Fire Brigade received the alarm at 3.30am and dispatched its new first-response rescue unit and five officers.
Brigade members, wearing splash suits and breathing apparatus, set up a containment area, assessed the scene, discovered leaking containers and stepped up the response, Mr Reeve said.
They were joined by five officers from Otematata fire brigade volunteers and members of the Waitaki Rural Fire Party, while four Dunedin firefighters specialising in dealing with chemical emergencies were flown to the scene by helicopter.
Two other firefighters from the Hazardous Materials unit (Hazmat) drove up in the Hazmat tender and brought a portable decontamination tent.
Police officers from Omarama, Cromwell and Alexandra also attended and put roadblocks in place at 8am.
They were not lifted until 5.45pm and more than 150 vehicles waiting at Omarama were sent on their way.
The cause of the accident had not been determined yesterday but weather conditions were among factors being investigated.
It was snowing and raining overnight on Tuesday and conditions remained poor most of yesterday, creating variable and cold conditions at the accident site.
Assistant fire commander, Southern region, Brendan Nally of Invercargill was in charge of the clean-up operation.
Mr Nally said the leaking containers posed a major risk to the environment and Lindis Creek.
"It happened half-way down a hill. Fortunately, the spill was not a significant size and there was no spill into the water," he said.
The Dunedin-based specialists wore fully protective clothing and breathing apparatus until the majority of the load was secured and the situation contained.
The firefighters then began working with Mainfreight and police to transfer the load by hand to another truck to be taken to Cromwell.
A crane was called to lift the truck and trailer unit upright.
