The freight train had roughly 819,000 liters (216,000 gallons) of the highly corrosive acid on board when it came off the tracks Sunday morning in a remote area of northern Queensland, state police said. They had initially said about 200,000 liters of acid was on board.
Officials believe that at least one of the freight cars has probably ruptured, which means as much as 31,500 liters of acid could have leaked out, according to a police statement
Authorities have declared an emergency and set up a 2-kilometer (1.2-mile) exclusion zone around the crash site. The exclusion zone is expected to remain in place until at least Thursday.
The train's locomotive ended up on its side, and all 26 freight cars it was pulling derailed, according to authorities.
Photos from the scene showed the jumble of cars sprawled across mangled tracks and waterlogged terrain.
Three men who were aboard the train were believed to have suffered minor injuries, police said.
Weather, floods hampering response
A nearby highway was closed as a result of the exclusion zone and flooding in the area.
It isn't clear what exactly caused the derailment about 20 kilometers (12.4 miles) east of the town of Julia Creek.
Police Inspector Trevor Kidd told the Australian Broadcasting Corporation
on Monday that there was soft mud at the crash site.
"You have to take into account it's remote, it's impacted heavily by weather, access is quite difficult, and these are the challenges the responders are facing at the moment," he said.
Initial testing by environmental officials suggested that the nearby Horse Creek waterway hasn't been polluted by the acid spill, police said Tuesday.