He said that between 60,000 and 90,000 tonnes of oil may have actually spilled into the sea, creating an environmental disaster of a totally different magnitude.
"It's enormous. That's at least as large as the official estimate of the Exxon Valdez disaster," Steiner said.
His comments come as Chinese authorities seek to move on from the accident, declaring the cleanup operation a "decisive victory".
Thousands of Chinese residents of the nearby city of Dalian have helped to remove the sticky crude from the sea, a strategy that the government says has worked.
But Steiner, who visited the oil spill area as a consultant for Greenpeace China, refutes the official line.
"It's habitual for governments to understate oil spills," Steiner told a news conference on Friday. "But the severity of the discrepancy is unusual here."
Steiner said he based his estimates on the fact that the oil storage tank that was destroyed in the explosion had a capacity of about 90,000 tonnes and reportedly had just been filled by the tanker.
He said his lower estimate of 60,000 tonnes came from the rate of oil recovery by the makeshift cleanup effort, which has involved thousands of fishing boats from nearby ports.
"They've already collected more oil than the official estimate of the spill size," Steiner said.
However, both Steiner and Greenpeace China warned their oil spill estimates could be up to 50 per cent off because of the lack of publicly available information about the incident.