The Department of Health and Human Services notified about 300,000 people that enrolled in health insurance plans through Healthcare.gov that Tuesday is the final day to verify they are eligible for Obamacare subsidies.

The Internal Revenue Service might require people to pay back those subsidies if they can't demonstrate through tax returns, pay stubs, letters from employers or other means that they qualify, the Wall Street Journal reported. Those payments would come at tax time next year.

Receive a notice? TODAY is the deadline to update your income info. Don't lose your premium tax credits. Call 1-800-318-2596 for support.

— HHS.gov (@HHSGov) September 30, 2014

Sept. 30 marks the end of the federal government's fiscal year.

Critics of Obamcare have expressed concern whether federal money is being doled out to only those poor enough to qualify for the subsidies.

The Journal reported this could affect anyone who may have gotten a raise during the year but didn't update the information with the government.

“Most people don't know they even got advance tax credits,” Mark Ciaramitaro, vice president, health-care services at tax preparer H&R Block, told the Journal. “They are going to be surprised and need to know what just happened, and a lot of people will be frustrated."

This March 1, 2014 file photo shows part of the website for HealthCare.gov, seen in Washington. (AP Photo/Jon Elswick, File)

On top of the 300,000, another 115,000 people could lose coverage obtained through the exchanges altogether on Tuesday because they didn't provide information about citizenship or legal residency status, the Journal reported."Generally, individuals who enroll through the marketplace and receive advance premium tax credits will file federal income-tax returns and, at tax time, advance payments of the premium tax credit will be reconciled," a Treasury Department statement said, according to the Journal.

Repayments could be capped between $300 and $2,500 for certain low income earners, according to the IRS.

The HHS Office of Inspector General found that the federal Obamacare marketplace sent notices to applicants asking to resolve inconsistencies, but the system did not have the capability to process inconsistent information.

At one point, the government couldn't determine the income of 1.2 million households. About 85 percent of the more than 8 million enrollees in private health plans obtained premium subsidies, according to HHS.