The Confederation of Australian Motor Sport (CAMS) said in a statement the accident occurred at about 1.50pm (AEST).
"On behalf of the motor sport community, CAMS offers its sincere sympathy to Peter's family and many friends," the statement said.
Brock was competing in the Targa West Rally at Gidgegannup about 30km from Perth.
Early reports state that Brock's car was so far off the road at the time of crashing that six competitors cars had driven past the crashed vehicle without noticing it.
Peter Brock was a dominant and well respected figure in Australian motor sports. Peter Brock won the Bathurst 1000, Australia's most prominent domestic motorsport event, a total of 9 times through the 1970s-80s.
Brock retired from full-time driving in 1997. And has since competed in motorsport events like the Targa Tasmania. he had made two return visits to Bathurst in 2002 and 2004 and returned to top-level touring car racing as a team owner of "Team Brock'' in 2002 in the V8 Supercar category.
A year later he sold his share in the team to Kees Weel.
In recent years, he occasionally competed in various motorsport events such as the Targa Tasmania.