Witnesses have described the terrifying moment a plane crashed into the Swan River in front of their eyes while they awaited the Skyshow display on the Perth foreshore.

Pilot and owner of the plane Peter Lynch, 52, and his partner Endah Aricakrawati, 30, died when the 1948-built Grumman Mallard plane crashed into the river, shocking thousands of people on the foreshore and on boats on the river nearby.

The City of Perth Skyshow event - billed as the most expensive in the country - was cancelled in the wake of the tragedy.

Acting WA Police Commissioner Stephen Brown was at the foreshore event preparing for a live television cross when he heard mass commotion.