The event, from Sept. 1 to 10, will bring international exposure to the sustainability needs of Hawaii, but at a price. Security alone for the event is expected to cost up to $4 million.
And that's money that the city wasn't planning for.
"There was no real proposal to say we need this much for security," said Honolulu City Councilwoman Ann Kobayashi, chairwoman of the Budget Committee.
She said there was money set aside for planning the event, but paying for it will mean other projects will have to lose out.
"We're honored to have the event here, we just need help with the financing," she said.
The congress will be the first major, international event in Honolulu since 2011, when world leaders gathered in the islands for the Asia-Pacific Economic Cooperation.
President Barack Obama and others came for the 21-member forum that promotes free trade throughout the region.
For APEC, the Honolulu Police Department alone spent about $10 million on security.
For the World Conservation Congress, the city has no security budget for the expected road blocks, police escorts, and monitoring.
"There are all kinds of expenses associated with an event like this," Kobayashi said. "For us, security is a real high priority and we want to make sure nothing happens when the event is here."
Colin Moore, director of the University of Hawaii's Public Policy Center, said the congress will spotlight Hawaii's environmental issues, and will pump a lot of money into the economy.
But, he said, it will put a burden on the city budget.
The state has appropriated funds security, in particular along the Ala Wai Canal.
"Unlike APEC, which qualified for a certain group of federal grants because the president was there and because there were so many heads of state there, that's not the case for the WCC," Moore said. "At this point, it's probably too late for the city to do much of anything. Taxpayers are going to foot a big part of this bill."
The mayor's office couldn't immediately provide the expect cost for security for the congress. A spokesman did say, however, that HPD alone will spend an estimated $455,000 on overtime for officers.