On the back of concerns raised by two drivers that going to work was "like playing Russian roulette", Fairfax Media can reveal that ACTION increased the weight limit for drivers by 10 kilograms in 2014.

One driver injured by a damaged seat in 2016 said removing the safety buffer was perplexing, and ACTION needed to start weighing drivers and enforcing its own weight limit policies.

Application forms for drivers from that year show that ACTION required drivers to weigh less than 120 kilograms, but recruitment information brochures distributed in 2015 and ever since say drivers must maintain a weight below 130 kilograms at all times.

The safe working limit for the ISRI 6500/517 seats installed in many ACTION buses is 130 kilograms, and ISRI seat instruction manuals contain a warning that failure to follow the manufacturers' specifications could result in serious or fatal injury.

ACTION does not weigh its drivers to ensure they comply with the specifications, and while some buses are equipped with driver's seats that can withstand up to 150 kilograms, there is no program in place to ensure that drivers who weigh more than 130 kilograms only drive those buses.

In response to questions, Transport Canberra and City Services, which is responsible for ACTION Buses, said the weight limit was increased in 2014 in line with new manufacturer specifications.

But Isringhausen, which manufactures the driver's seats in ACTION buses, said the weight limits had never changed.

"The 6500/517 seats have always been 130 kilos," said Kavi Naicker, from Isringhausen's sales department.

Two worried drivers spoke out last month about concerns that overweight drivers were damaging seats, creating a risk that they could "bottom out" or come loose mid-trip, putting drivers and passengers' safety at risk.

One of the drivers, who did not want to be identified, has already been injured.

He had to take more than a year off work after a damaged seat "bottomed out" during a test run that he was directed to undertake in 2016, after raising concerns about the seat three days earlier.

Comcare accepted liability for the driver's lower back injury, which was described as an aggravated lumbar sprain.

The injured driver said the increase in the weight limit was perplexing because it removed a 10-kilogram buffer zone, leaving no room for error.

"We started raising these issues in 2014 and all of a sudden they changed [the weight limits]," he said.

"You can't be red-lining any mechanical device's safe working limits. There needs to be risk mitigation."

While he was disappointed by the increase in the weight limit, the injured driver said he was more worried by the fact that ACTION did not weigh its drivers to enforce either the old or new limit.

"I find it quite perplexing that any organisation would take the risk to allow this kind of hazard to continue in the workplace when there's a risk of serious or fatal injury," the driver said.

"You've got a mechanical device with a manual that tells you what weight it can handle, and you should stick to that, 100 per cent, or the ramifications could be dire.

"It's a black and white issue. It's quite simple, really."

Fairfax Media asked Transport Canberra and City Services whether ACTION should have to weigh drivers, given the warnings in the seat manual that failing to follow the instructions could result in serious or fatal injury.

The manual also warns that if the specifications are not followed, "the liability, warranty and possibly the general operating permit will be lost".

A Transport Canberra spokesman said in an emailed response that the directorate made every effort to ensure the safety and wellbeing of its drivers and the public, but did not address specific concerns that current policies could put drivers and the community at risk.

The spokesman also said ACTION buses travelled about 26 million kilometres a year and had a strong safety and maintenance record. He said seats were periodically refurbished and replaced.

All drivers were required to undergo an annual medical assessment to confirm they were medically fit to do their job safely, according to the spokesperson.

Drivers have told Fairfax Media that test does not include being weighed.