The company pleaded guilty in the Dunedin District Court last month to two counts of operating an unsafe vehicle and was fined $1800.
First Union southern region secretary Paul Watson said that fine was ''woefully inadequate'' and sent the wrong signal to the transport industry.
''It beggars belief that they could impose such a low fine.''
The union did not represent Summerland employees, but had a collective agreement with hundreds of other transport companies in New Zealand.
A pair of Summerland truck crashes at the beginning of this year, both of which occurred early in the morning, were found by police to have been caused by impaired and careless drivers rather than mechanical issues.
The first happened on January 3 on State Highway 1, south of Palmerston, when a truck and trailer crashed off the road at 3am, leaving groceries strewn across the side of the road.
The 51-year-old driver was taken to hospital with an arm injury, and was later charged with both careless driving and drugged driving.
Eight days later, another large Summerland truck rolled about 4.40am on State Highway 1, near Tinwald, south of Ashburton.
The uninjured driver was charged with careless use of a motor vehicle.
Mr Watson said these crashes ''start to raise questions about drivers' work pressure''.
He had heard anecdotal evidence about truck drivers taking illegal stimulants such as speed or methamphetamine ''because of long hours and pressure'', but said it was difficult to say for certain if drugged driving was a major issue in the trucking industry.
A source who knew several Summerland truck drivers alleged they were ''pushed to the limit'' by management.
The latest crash of a Summerland truck to trigger a police investigation happened on Sunday, on State Highway 1, between Waipahi and Gore.
A police spokeswoman said they were investigating to ''determine cause and any fault to be attributed to any party'' after the Summerland truck crashed while travelling from Invercargill to Dunedin.
Staff at Summerland's Dunedin branch referred requests for comment to general manager Grant Lowe.
He did not answer repeated calls or respond to messages left with staff at his Christchurch office.
A NZ Transport Agency spokeswoman said it was aware of the recent spate of crashes and was ''working with the company's managers to ensure that they are aware of their responsibilities''.
Associate Minister of Transport Julie Anne Genter declined to comment on whether an $1800 fine was appropriate for a company operating an unsafe vehicle that left an employee with critical injuries, but said penalties for traffic safety offences were to be reviewed.
''The Government is developing a new road safety strategy and part of this work will consider whether penalties around road safety are appropriate.''
