About 100 Veolia Transport drivers are planning to take industrial action from 4am (AEST) on Thursday until 8am on Friday, affecting services in Redland Bay and Brisbane.
Transport Workers Union (TWU) bus coordinator Bob Giddens said conditions for Veolia drivers were poor.
"The timetables are so tight that the drivers are constantly running late and the concern there is that the drivers tend to speed up and work hard to try and stay on time," he told AAP.
"They're putting their licence at risk every day they're out there."
Veolia bus driver and union delegate Greg Packer said staff were working excessive hours to supplement earnings of $760 per week before tax.
"They'd be doing anything from 20 to 25 hours on top of their 38 hours, it's pretty common," he said.
Drivers didn't have toilet or lunch facilities, and were forced to eat and relieve themselves by the side of the road, he said.
"If you've got 25 people or 30 people on your bus, and you have to say to them 'excuse me I need to pull over at a service station', it's hard. It's embarrassing. And it also makes you run late."
He said he'd heard a story of a female driver who had gone to the toilet in bushes and been interrupted by a black snake.
Supervisors were not sympathetic when she complained, he said.
"She was told that maybe she should have used a bottle like the guys do," he said.
But Veolia Transport managing director Colin Jennings said the package being offered to workers, including a pay rise of about four per cent, was fair.
"We believe ... that's a fair outcome given the economic conditions that we're facing here in Queensland and Australia at the moment," he told AAP.
The Transport Workers Union will meet with Veolia Transport on Wednesday afternoon in a last-ditch bid to avert the strike.
Thousands of passengers, including students at 60 schools, will be inconvenienced if the strike goes ahead.
Veolia school buses in the Redlands and Capalaba area won't run on Thursday and Friday.
Commuter services to Brisbane from Redlands and the bayside suburbs will run as normal between 5am and 9am, and 4pm and 6pm (AEST), and a Saturday timetable will apply for local and off-peak services.