The organisation was forced to slaughter fish and birds affected by water from Optimum Coal's mine, Middelburg SPCA manager Petro Oosthuizen said in a statement.
"[Previous] investigations confirmed that the deaths of thousands of fish and the secondary poisoning of birds resulted from contaminated water being pumped into the dam by the mine.
"There were thousands of dead fish and large numbers in the throes of death whose suffering was mercifully ended."
The SPCA had to obtain a warrant to get access to the Eikeboom pan, which is on privately-owned land, when they were denied entry on March 7.
"It is evident that the mine had both the technical knowledge as well as the means to prevent this from occurring, instead they chose to try and prevent us from assisting dying animals," Oosthuizen said.
She said Optimum Coal used the Eikeboom pan for discharging mine water and had done so for over 15 years.
Middelburg police could not immediately be reached for comment. – Sapa