Officials said on Saturday that the tanker had tilted further and sank deeper into the sea, causing the fresh spillage.
"It was a pretty bad day," Brigadier Iftikhar Arshad, a senior official of the Karachi Port Trust said.
The tanker, Tasman Spirit, carrying 67,000 tonnes of crude had earlier broken into two on August 14 after being grounded off the Karachi port on July 27.
It had spilled around 12,000 to 15,000 tonnes of crude into the sea, killing marine life and polluting the sea.
Alarmed by its catastrophic consequences, the authorities had swung into action to contain the spill.
But the fresh spill is now threatening to undo the clean up drive.
"We are using booms and skimmers to contain oil, while a C-130 aircraft is spraying chemicals on the crude to help it settle on the bed," an official said.
Authorities have so far drained 30,000 tonnes of crude. But 20,000 tonnes are still in the ship.