He estimated Georgia drivers would pay an added $400 million annually in premiums.
He hand-delivered a letter Tuesday to the members of the House Judiciary Non-Civil Committee that's considering the proposal.
"It really concerns me," Mr. Oxendine said. "It's a prime example of the Legislature saying, 'We really want to do something.' It sounds good, but they haven't thought about what the effect would be."
Sen. Cecil Staton, R-Macon, sponsored Senate Bill 276, which passed the Senate 46-3 on March 27. It would change the way insurance companies determine how much they pay out in accidents involving underinsured motorists.
If a driver with the required minimum $25,000 in liability coverage causes more than that in damages, then the insurance of the car that's hit, with so-called Uninsured/Underinsured Motorist coverage kicks in.
Currently, that coverage would pay the policy's limit minus the amount paid by the insurance of the driver who caused the accident.
Mr. Staton's bill would require payment up to the policy's limit in addition to the payout from the driver who caused the accident.
Since the insurance companies could be shelling out much more money, they'll want to raise their premiums to cover their risks, the commissioner said.
"In my analysis, the minimal benefits of S.B. 276 would be greatly outweighed by the increased costs to Georgia consumers," Mr. Oxendine wrote.
He notes in his letter that 84 percent of all vehicles have Uninsured/Underinsured Motorist coverage. He wrote that his cost estimate came from conversations with the state's largest car insurance companies and analysis of information from the National Association of Insurance Commissioners Auto Insurance Database Report.
He estimates premiums for Uninsured/Underinsured Motorist coverage would jump 150 percent.