The spill is was times as big as initially estimated.
A fishing boat in Thailand that had apparently been hired to transport fuel sank, dumping nearly 10,000 litres of fuel in the mouth of the Tah Chin Klong, off Koh Sirae on Friday, local newspaper the Phuket News reports.
Officials from the Phuket Marine Office said the amount leaked was nearly five times the original estimate of 2,000 litres.
"We underestimated it," Puripat Teerakulpisut, Phuket Marine Office Director, told the newspaper.
"We initially estimated the leaked fuel was about 2,000 litres. But there was almost 10,000 litres, so it took three days to clean it up - we finished on Sunday."
Officials said whoever is responsible may face charges, but it is not yet clear who that is, since they believe someone hired the boat to transport the fuel.
Puripat said authorities removed 20 tanks of thick oil, and they saw little effect from the spill on wildlife or marine animals.
Puripat Teerakulpisut, Phuket Marine Office Director
But there was almost 10,000 litres, so it took three days to clean it up
"There were many areas that the fuel drifted into," he said.
"We put booms in the klong on the first day of the spill, but during the night when boats tried to moor, some hit the boom and fuel spilled out.
"This made our job tougher and it took a lot of time."
Authorities are now testing water samples from the area and expect results within 15 days.
Immediately after the spill, the Phuket News quoted Phuket Marine Police Deputy Superintendent Lt Jeerayuth Onthong, who said the crew abandoned the boat.
"Then they left the boat and never came back again," he said.
In the more recent article, the newspaper reports that authorities still have not questioned the boat's owner.