Owners and managers of Hawaii vacation rentals must post their transient accommodations tax identification number on their online advertisements or face stiff fines under a new law that took effect Friday.

Act 204, introduced by state Sen. Laura Thielen and signed into law by Gov. David Ige last year, aims to crack down on operators of vacation rentals who aren’t paying their taxes and to give the counties an added tool to rein in the thousands of illegal vacation rentals that have proliferated throughout the islands.

The counties have worked to shut down illegal vacation rentals on and off over the years. But websites such as VRBO, AirBnB and TripAdvisor has made it easier for property owners or lessees to rent out their homes and apartments on a short-term basis and harder for the counties to regulate the industry.

With the state, and Oahu in particular, facing a housing shortage, Thielen says the law could add needed inventory to the housing market.

Hawaii’s shortage of rental housing is expected to grow to more than 20,000 units by 2020, according to statistics from the Department of Business, Economic Development and Tourism.

“At the same time we have had a huge conversion of residential homes in our state to short-term vacation rentals,” said Thielen. “This is an issue of housing for local residents.”

More than 22,000 vacation rentals were being advertised online, according to a 2014 report by the Hawaii Tourism Authority — and only a fraction of these are believed to be legal. For instance, the study found 4,411 vacation units were being advertised online on Oahu at a time when there were only 828 permitted vacation units, according to data from Honolulu’s Department of Planning and Permitting.

In addition to affecting the housing supply for local residents and driving up rents, housing and tourism experts say the state is likely losing millions of dollars annually in revenue because not all vacation rental owners are paying their transient accommodations tax — a 9.25 percent tax on gross revenue from short-term rentals, or units rented for less than 180 days.

The exact hit on transient accommodations tax revenues, which reached $421 million in the 2015 fiscal year, is not known. Hawaii Department of Taxation spokeswoman Mallory Fujitani said the state doesn’t have an estimate on how much revenue it might be losing every year. She said the department also doesn’t have a breakdown on how much of the transient accommodations tax revenue comes from vacation rentals versus hotel rooms.

Under Act 204, vacation rental operators who don’t post their transient accommodations tax number on their online advertisements can be fined $500 a day for a first-time violation, $1,000 a day for a second violation and $5,000 a day thereafter.

Vacation rental operators not residing in Hawaii or on the island of the rental also must post a local contact on the ads or face the same fines.

“It does provide us with an additional tool to help identify operators of short-term rentals,” said Fujitani, though she stressed that the Department of Taxation is not concerned with whether the rental is legal or illegal. It will be up to the counties to use the information to help enforce rental regulations.

Anyone can get a transient accommodations tax number. Thielen plans to introduce a bill this year, however, that bars the Department of Taxation from issuing a transient accommodations tax license unless the vacation rental operator can show they have a county permit. “I don’t see why we should be issuing … licenses to illegal vacation rentals,” she said.

Fujitani said the Department of Taxation is concerned only with making sure people pay their taxes. “That is the area we have jurisdiction over, whether people are reporting or not reporting revenues,” said Fujitani. “The land-use side is something we don’t know anything about. If they are doing an illegal rental, we still expect them to pay their tax.”

The Department of Taxation was given $200,000 by the Legislature to help with enforcement. But it will likely be a few months before the department begins handing out citations to vacation rental operators, said Fujitani. She said the department plans to focus on education efforts first.