The state Department of Transportation erred by looking only at the Hawaii Superferry's potential affect on Kahului Harbor and not at its overall impact on Hawai'i's waters, the state Supreme Court said.

The five-member Supreme Court ruled last week that the state had to conduct an environmental review of the ferry service, in a move that resulted in the suspension of the Superferry's Honolulu to Kahului service.

Yesterday, the high court issued a 104-page opinion explaining its decision.

"The exemption (to the environmental study) was erroneously granted as the DOT considered only the physical improvements to Kahului harbor in isolation and did not consider the secondary impacts on the environment that may result from the use of the Hawaii Superferry," wrote Associate Justice James Duffy, who authored the opinion.

"Stated simply, the record in this case shows that DOT did not consider whether its facilitation of the Hawaii Superferry Project will probably have minimal or no significant impacts."

In 2004, the state Transportation Department decided an environmental assessment of the Superferry's impact would not be needed because the only structural changes were in harbors that are not environmentally sensitive.

Duffy's conclusions echoed arguments by environmental groups — including the Sierra Club, Maui Tomorrow and the Kahului Harbor Coalition — who sued the state in 2005 over its handling of the Superferry.

"This means the state has to look at a project as a whole and not just at the isolated parts" when it exempts someone from having to do an environmental review, said Isaac Hall, attorney for the three groups.

Hall noted that the type of environmental assessment required by the Supreme Court could take months to complete. If the assessment finds problems, the state may have to conduct a full environmental impact statement on the Superferry that could delay the ferry for up to a year.

Hawaii Superferry officials did not respond to calls seeking comment yesterday.

Department of Transportation officials also had no comment and Deputy Attorney General Bill Wynhoff, who has argued the case on behalf of the DOT, did not return calls.

Gov. Linda Lingle previously defended the state's handling of the issue, saying the Superferry was being unfairly singled out because airlines, barges and cruise ships have not had to go through similar environmental scrutiny.

The Supreme Court's opinion caps a tumultuous first week for the ferry service.

The company launched its maiden Honolulu to Kahului voyage on Sunday with one-way $5 fares. On Monday, Kaua'i protesters blocked the entry of the company's $85 million vessel into Nawiliwili Harbor, forcing the ship to turn back to O'ahu.

The company indefinitely suspended its Kaua'i service on Tuesday at the urging of Lingle.

About 14 protesters have been arrested since Monday.

Also on Monday, Circuit Judge Joseph Cardoza issued a temporary restraining order halting the Superferry's service to Maui due to last week's ruling by the Supreme Court.

Superferry officials said its Maui service will remain on hold until Thursday at the earliest.

Reach Rick Daysog at rdaysog@honoluluadvertiser.com.