Summerland, meanwhile, has said its safety was improving. One leading tracking system showed the company's fleet and drivers were among the most safe.
The truck-and-trailer unit was heading south carrying freight and timber when it rolled off the Lewis Pass Rd (State Highway 7) shortly before 11pm, west of the turn-off to Hanmer Springs.
A police spokeswoman said the vehicle was off the road but the driver was not trapped and was out of the truck on the arrival of emergency services.
He was taken to the Hanmer Springs Health Centre for a check-up but was later airlifted to Christchurch Hospital in a serious condition, according to police.
In a media release, Summerland General Manager Grant Lowe described the driver's injuries as moderate.
No other vehicles were involved, he said.
Mr Lowe said local police were investigating but the police Commercial Vehicle Safety Team (CVST) was not involved.
He did not want to comment further on the crash as the NZ Transport Agency was investigating earlier incidents involving the company.
"The recent incidents are disappointing, but they don't reflect the constant improvements we've been making,'' he said.
Mr Lowe said a tracking system showed the safety of the company's fleet was improving.
"For example, one [of] New Zealand's leading fleet tracking systems, EROAD, has been tracking truck speed in 2700 heavy transport organisations.
"In 2017, the Summerland fleet was in the top half for safety; in 2019 it is now in the top 15%, with over a third of Summerland drivers in the top 10%.”
Trucks belonging to the Cromwell-headquartered company have been involved in at least five crashes in the South Island since December which have spurred investigations by the CVST.
In March, former Summerland truck driver Vasile Urechiatu, who quit after repeatedly working more than 70 hours a week, came forward to the Otago Daily Times to say he feared he was going to cause a serious crash.
Originally from Romania, Mr Urechiatu said he repeatedly worked illegal shifts of more than 14 hours, while the transport firm conceded it made mistakes in allowing him to break work-time rules.
He came to New Zealand to work for Summerland and enjoy fishing and hunting, but his dreams were dashed when he left the company which jeopardised his immigration status, and he now lives in England.
NZ Transport Agency Southern road compliance manager Kelvin Lloyd, of Dunedin, said police were leading the investigation into the claims by Mr Urechiatu, though the agency was working with police on that and other investigations.
