Bus company Pacific Tourways has laid a police complaint against the passengers and the tour operator who allowed the group off to relieve themselves in public.
An Ashburton resident posted pictures of the group on social media.
They said on Friday the bus had pulled over in Tinwald on State Highway 1 outside the medical centre, and let its passengers out who "proceeded to urinate on the road side".
"I am absolutely disgusted.
"Men stood along the road while women went into the trees with toilet paper which I can only guess would still be there," the post said.
There were two sets of public toilets within one kilometre, they said.
The post has been shared over a thousand times and drawn dozens of outraged comments.
"What is wrong with people there are toilets in town the domain," one person said.
"There was a tour group on the bus and they let the passengers off, not the driver. They are the ones responsible. If the driver is told to stop by the tour group they stop, the driver is not in control of what the passengers do.
"There is an investigation happening and we have laid a complaint with police against the passengers.
"We don't condone this kind of behaviour."
A police spokeswoman said the tour group could possibly be charged with excreting in public, but this depended entirely on the circumstances.
