New Jerseyans rushing to their town halls this week in hopes of prepaying next year's property tax bills before the new federal tax law kicks in Monday just got a last-minute boost from Gov. Chris Christie.

But the IRS may be limiting how big of a boost they'll get.

Christie issued an executive order Wednesday requiring that all municipalities in New Jersey permit homeowners to prepay 2018 property taxes, as long as the payments are postmarked by the end of the year, which is Sunday.

The move is designed to help taxpayers temporarily minimize the impact of the federal law signed last week by President Donald Trump, which will cap the amount in state and local taxes that homeowners can deduct from their federal income taxes at $10,000 beginning Jan. 1. That's less than half of what an average homeowner claims in New Jersey, home to the nation's highest property taxes.

The idea is that by prepaying now, homeowners can can write off the payments on their 2017 federal income tax returns and cash in on the full deduction one last time.

Many municipalities -- from Jersey City to Hoboken to Evesham -- had already been accepting at least partial prepayments from worried residents.

But Christie said not every town in the state was following suit. His order instructs the director of the state Division of Local Government to mandate that all of New Jersey's 565 municipalities do so.

"The action I took today will ensure that local governments are flexible and accommodating of their local property taxpayers as we transition to the new federal tax code for 2018," Christie said in a statement. "This executive order requires local officials to dedicate the resources and staffing to serve New Jerseyans who are planning in this way for their families and their futures."

There's a snag, though. The Internal Revenue Service said Wednesday that 2018 prepayments are deductible only if you've already received a bill from your local government and paid it by Sunday. That could add confusion for those hurrying to meet the deadline.

Experts say New Jersey's tax bills have been sent out only for the first and second quarters of 2018. That may mean homeowners are allowed to deduct prepaid taxes only for the first half of next year but not the latter half.

"You can't take a deduction by remitting a check now for an anticipated future tax," said Frank Baglieri, a founding partner at Murphy, Miller & Baglieri LLP, a Glen Rock accounting, tax and consulting firm. "You know you will owe the tax and can probably make a very good estimate, but you have not yet been billed."

Still, Judson M. Stein, a lawyer with the Newark law firm Genova Burns LLC, said there "may be a reasonable basis to assert" that third- and fourth-term payments should be deductible.

Neither Christie's office nor the state Department of Community Affairs immediately returned messages seeking comment.

How many New Jersey homeowners would benefit from Christie's order is also unclear.

The move will "save thousands of dollars for countless New Jersey families," said state Senate President Stephen Sweeney -- a Democrat who last week called on Christie, a Republican, to issue such a directive.

Sweeney, D-Gloucester, also said Christie's order will give municipalities a cash windfall to "invest" and benefit taxpayers.

But Mike Cerra, assistant executive director of the New Jersey League of Municipalities, said the pool of people "able to take advantage of this is probably relatively small."

Trump and his fellow Republicans who control Congress say the new law will cut taxes for people across the country.

But while there is currently no limit on how much its homeowners can claim in property taxes on their returns, the $10,000 cap on the total deduction for state and local property taxes and income or sales taxes will leave many people in high-tax states like New Jersey claiming much less than they do now.

Property tax bills alone top $10,000 for about 25 percent of single-family homes in New Jersey, according to ATTOM Data Solutions.

With time running out, residents across New Jersey and other states have flooded their local tax offices in recent days to prepay their property tax bills. You can't prepay income taxes.

"We are getting a ton of phone calls, a ton of emails, and a lot of traffic in person," Montclair tax collector Lidia Leszczynski told NJ Advance Media in a phone interview Wednesday. "Absolutely nonstop."

Friday is the last day most government offices are open until the new year, though some New Jersey municipalities -- including Montclair -- are opening their tax offices Saturday to accommodate residents.

The New Jersey Society of CPAs cautioned that homeowners should consider a few factors before prepaying: Make sure you have enough money in your accounts; those subject to the Alternative Minimum Tax will not benefit; and Congress could act in the coming weeks to retroactively reject prepayments.

Christie's move is similar to an emergency order New York Gov. Andrew Cuomo issued in his state Friday that taxpayers there to prepay their 2018 taxes.

For Christie, the order is notable because while he's close to Trump, he also governs one of the states that will be hit the hardest by the tax overhaul.

Christie, who leaves office Jan. 16, has also called for the Democratically controlled state Legislature and incoming governor Phil Murphy, a Democrat to change New Jersey's tax deductions to make property taxes completely deductible on state income tax returns.

"We can do something to fix this, and we should," Christie told reporters last week, adding that property values in the state could fall if such a move isn't taken.

NJ Advance Media staff writers Claude Brodesser-Akner, Samantha Marcus, Jonathan D. Salant, and Karen Yi contributed to this report.

Brent Johnson may be reached at bjohnson@njadvancemedia.com. Follow him on Twitter @johnsb01. Find NJ.com Politics on Facebook.