Honolulu is prepping for a national gathering of mayors. But who’s footing the bill?

For the first time since 1963, the nation's mayors will head to Hawaii for an annual conference

HONOLULU, Hawaii (HawaiiNewsNow) - The U.S. Conference of Mayors will be held in Waikiki in June. It’s expected to cost about $5 million to put on.

Taxpayers will cover about $100,000 of the bill, as approved by the Honolulu City Council; however, a city spokesperson says it does not expect to use all of that money.

More than $3.3 million will come from private donors -- and that’s raising some red flags to some who worry about conflicts of interest.

Much of that money came from city contractors and subcontractors or from companies like AirBNB and Uber who are fighting hard against City Council measures to regulate their businesses.

“I think there are some concerns" said Colin Moore, University of Hawaii political science professor. “Sponsors who do business with the city or are involved in rail projects ... you have to wonder what they’re getting as a result of the sponsorship.”

The city said the sponsorships pose no ethical problems.

“The City and County of Honolulu has proper protocols and procedures in place to ensure such support does not create conflicts of interest," said city spokesman Andrew Pereira.

According to the conference website, donors include the engineering firm AECOM, which contributed at least $150,000 and Kiewit Pacific and Stantec , which contributed more than $25,000 to the event.

All three companies work as contractors or subs for the controversial Honolulu rail project.

Other big donors include AirBNB, Expedia, Lyft and Uber. These companies have long opposed city efforts to regulate their industries.

“One would have to look very carefully to see if these donations are being made to influence the city in future policies," said Ian Lind, investigative reporter and former Common Cause director.

The conference will be held at the Hilton Hawaiian Village from June 28 to July 1.

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