In response to climate change rallies in Adelaide this week, the state Labor government rushed legislation into parliament increasing the maximum fines for public obstruction from $750 to $50,000 or a three-month prison sentence.
The changes have the support of the Liberal opposition ensuring their swift passage through both houses.
But SA Unions said the increased fines were designed to intimidate people out of challenging those in power.
"The government is telling South Australians that public assembly and protest are only legitimate when it is convenient for the people in power," SA Unions Secretary Dale Beasley said.
"Our rights to assembly and protest exist so that people can disrupt injustice. Disruption is the point.
"Peaceful but disruptive protests and demonstrations have been integral in achieving so much of what we now take for granted in our society."
The Conservation Council of SA said the new laws were intended to have a chilling effect on community action.
"Throughout history, direct action and street protest has been an essential step towards changing society for the better," CEO Craig Wilkins said.
"By its nature, blocking streets and holding rallies is intended to be disruptive and raise attention and debate."
Introducing the legislation on Thursday, Premier Peter Malinauskas told parliament there were few things more important than the freedom of association and the ability to protest peacefully.
"It is an essential formulation to the way we govern ourselves. It's an essential component of the functioning of our democracy," he said.
"But that is a very different thing to someone or a group of people who conduct themselves in such a way that puts other people's safety at risk."
The government's move came after three days of protests by members of Extinction Rebellion, including the action of a 69-year-old woman on Wednesday who abseiled over a city bridge and disrupted peak-hour traffic for about 90 minutes.
Opposition Leader David Speirs said that proved costly for businesses and there were reports people had been delayed attending appointments at the nearby Royal Adelaide Hospital.
"These types of protests are getting out of control and we are sick and tired of seeing groups and individuals receive nothing more than a slap on the wrist," he said.
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