Tax day is here—and some people will pay more this year because of Obamacare.

The law's biggest tax provision—billions of dollars in tax credits to help people cover the cost of their premiums—is already in effect, but doesn't affect the taxes due on Tuesday. A handful of smaller provisions, mostly affecting wealthy households, will show up for the first time in this year's filing.

Among this year's changes: a 0.9 percent increase in Medicare taxes and a 3.8 percent surtax on investment income. Both are limited to high-income taxpayers, and both took effect for the first time in the tax season that just ended.

Most people won't notice the extra Medicare tax because it was automatically deducted from their paychecks, but some could face a tax bill they did not expect, said Jackie Perlman, principal tax research analyst at the H&R Block Tax Institute.

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