The Greek-registered MV Tasman Spirit split into two some 100 meters from the Karachi port and spilled at least 7000 tonnes of its liquid cargo.
The ship had been berthed off the port for a fortnight after running aground on 27 July.
Before the ship broke up, some 20,000 tonnes of its 62,000 tonne oil cargo had been transferred from the ship in a salvage operation that was abandoned on Wednesday when cracks widened and the ship began to sink.
"It is our assessment that about 35,000 tonnes of oil is in the ship and we have unloaded 20,000 tonnes already and the rest you can calculate," a Pakistani official said.
The oil spill triggered immediate damage, with dead fish and lifeless sea turtles washing up the shores of Karachi.
But senior officials played down the catasthrope and insisted the disaster has been contained.
"The worst is over now and despite the ship being split, all remaining oil is intact and there is no more spillage from it," they said.
'It is our assessment that about 35,000 tonnes of oil is in the ship and we have unloaded 20,000 tonnes already and the rest you can calculate'
Karachi Port Trust chief Ahmed Hayat said the remaining oil would be unloaded once the ships' parts had settled into the sea bed.
But predicting nightmarish consequences, environmentalists said more needed to be done.
"A 16 km patch of the eastern shore has been badly hit by the spill," Ahmed Saeed of the International Union for Conservation of Nature said.
Saeed said the spill could result in a deadly blow to marine life. "The affected area is the habitat of small fish and we fear that either they will die or migrate. That would be an irreparable loss," he said.
Environmentalists also expressed fears that heavy monsoon winds could push the slick eastwards and affect even the mangrove forests at a distance.