Investigations on Saturday showed the vessel had disintegrated into three pieces, Cape Town city disaster management spokesman Wilfred Solomons-Johannes said.
"It is believed the rough seas and high spells that were experienced yesterday (Friday)... resulted in the movement of the vessel and caused the oil spill from the vessel," he said.
Aerial investigations "established that a thick layer of bunker oil has washed ashore and that large quantities of oil is still in the water in the vicinity of the SELI 1 vessel that ran aground on 07 September 2009," he said.
"Black oil fingers" of up to 500 metres radius originating from the vessel was trickling toward the city's Dolphin Beach, he added.
Efforts to clean the oil were suspended until Monday due to rough seas.
The Panamanian-registered, and Turkish-operated ship was carrying 30,000 tons of coal and 600 tons of heavy fuel when it developed engine problems in September 2009 while on its way to Gilbraltar
It has since remained there and South African authorities require 40 million rand ($4,7 million, 4 million euros) to clear the wreckage.