In the course of little more than a year, Richard V. Allen has gone from President Reagan's national security adviser to foreign agent.

Allen was ousted from office last year after disclosures that he accepted $1,000 from a Japanese magazine and three Seiko watches from a Japanese businessman.

Allen's ties to the Japanese are now more formal and far more lucrative. According to government records, Allen recently signed a $300,000-a-year contract to serve as chief lobbyist for a consortium of Japanese companies, including Mitsubishi and the Bank of Tokyo, which are seeking U.S. government approval for a new sea-level canal through the Isthmus of Panama.

In that capacity, Allen has tapped his administration connections. When a group of his Japanese clients arrived in town last winter, Allen arranged meetings for them with Treasury Secretary Donald T. Regan, Assistant Secretary of State Thomas O. Enders and other high-level government officials.

The meetings raised questions at the State Department, where some officials wonder about the propriety of a former government official who once had access to classified information being on a foreign payroll.

Allen's reaction is that "this is not a matter that vitally affects the national security. I'm very much aware of seemliness and propriety."

Allen's judgment is shared by dozens of former government officials, including ex-senators, Cabinet secretaries and agency directors who are selling their expertise to foreign governments and corporations.

Justice Department officials say that the foreign "revolving door" is moving so briskly they can no longer police it.

Just last month, for example, former senator Charles Goodell (R-N.Y.) registered as a lobbyist for a French aerospace firm.

Stuart Spencer, a longtime political consultant to Reagan, registered to handle public relations for the government of South Africa.

Other recent filings at Justice show:

* Former defense secretary Clark Clifford's law firm received $105,000 last year from the Australian Meat and Live Stock Corp. to lobby Congress on farm import legislation and $32,400 from Atlantic Container Lines Ltd. to lobby the Defense Department to buy the British firm's container ships.

* The consulting firm of William Colby, former director of the CIA, received about $180,000 last year for political and economic analysis provided to the Government of Singapore Investment Corp.

* The law firm of former transportation secretary Brock Adams received about $483,000 last year to lobby on behalf of a variety of foreign transportation clients, including the China Ocean Shipping Co., the Japanese Fisheries Association, the Japan Deep Sea Trawlers Association and the Hokuten (Japan) Trawlers Association.

To monitor such activities, the Justice Department has a 17-person unit whose job it is to ensure compliance with the Foreign Agents Registration Act. That law was passed in 1938 because of concern about Nazi propagandists and other subversives.

Critics say the law is weak and enforcement is sporadic. There hasn't been a criminal prosecution under it since 1963. "The act has never been given very much attention by the Justice Department," says Joel Lisker, the former registration chief who is now staff director of the Senate Judiciary security and terrorism subcommittee.

"Many people who occupy senior positions at justice, if they haven't represented foreign principals themselves, associate with law firms that do. There's a tendency for them to look ahead to what they may do" after they leave government.

Although the law requires all foreign agents to file semi-annual reports disclosing their activities and fees, Lisker says that Washington law firms with foreign clients often hide their public relations and lobbying activities behind the shield of "attorney-client" privilege.

"Voluntary compliance is our main form of enforcement," said Joseph E. Clarkson, who now heads the registration unit.

In the past five years, the number of registered agents has grown from 5,200 to 6,700, an increase attributed in part to the perceived success of the Israeli and Greek lobbies in influencing U.S. policy.

But Clarkson says, "There are probably 30 to 60 percent more agents out there that are not registered as are."

The loose enforcement leads to anomalies, particularly in the national security area. Last month, Reagan kicked up a storm when he signed an executive order requiring all federal employes with sensitive security clearances to get approval--even after leaving the government--before publishing anything that might contain classified information.

Justice officials concede there is no way to require a former government official to get prior approval before privately divulging information to foreign governments.

A former official "who knows all the secrets can couch his advice to take advantage of that, it goes through that filter . . . . He has the benefit of inside thinking on a variety of issues," Lisker said. "This is what makes him so valuable."

But others say the disclosure problem is exaggerated. "The various messages that I send to a foreign principal are here, and anytime the government wants to come take a look at them, they can," says Colby. "I still feel bound not to tell anybody any classified information."