"We have obtained documents which in our opinion demonstrate that the (police) version of the events at Marikana... Is in material respects not the truth," the commission said after gaining access to police hard drives.
Police say they were acting in self defence.
The commission also accused police of falsifying documents, hiding documents, lying about the existence of other documents and giving a false account of events.
"We have obtained documents which give the impression that they are contemporaneous documents, but which appear in fact to have been constructed after the events to which they refer," the commission said.
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A spokesman for the police ministry declined to comment.
"It's before a commission of inquiry, that is sub judice," Zweli Mnisi told AFP.
The damning statement against the police is unusual, since the commission has not yet wrapped up its work.
"We do not make this statement lightly," it said.
The commission said it will adjourn until next Wednesday to enable investigators to trawl through the "thousands of pages".
It took note the police members had handed over the information willingly.
"The judge clarified the matter to say that these are not findings of the commission, it is evidence of some wrong-doing that the investigators have come across."
President Jacob Zuma appointed the commission to investigate the police killing of 34 striking miners on August 16 last year at a mine run by platinum giant Lonmin.