BANGKOK, JAN. 28 -- Thailand's marine police pushed a boatload of about 40 Vietnamese back to sea yesterday in an attempt to stem an increasing flow of refugees into the eastern coastal provinces, western diplomats and refugee officials said here today.

Thai newspapers reported that the Vietnamese, who arrived Tuesday in a fishing boat from Cambodia, were sent back to sea after local officials discovered that they had entered Thailand illegally with the help of smugglers.

Senior Thai officials, police and Navy officers met in the coastal town of Khlong Yai yesterday to discuss ways to slow the growing number of Vietnamese boat people coming to Thailand via Cambodia. Officials said local police have been bribed to look the other way when professional smugglers bring refugees into Thailand.

The number of boat refugees arriving in eastern Thailand began increasing dramatically last October, when 1,215 Vietnamese landed, according to statistics gathered by the U.N. High Commissioner for Refugees. In December, 2,300 came.

About 11,200 Vietnamese refugees reached Thailand last year, a total nearly three times higher than the 1986 figure of 3,886.

"Thailand clearly wanted to show for domestic consumption that it is heroically stemming the tide," one refugee official said, "and send a signal to Vietnam that Thailand is not a great way to come."

Thai officials are also frustrated with the growing number of Hmong, an ethnic minority, arriving from Laos. National Security Council Secretary General Suwit Suthanukul estimated that 10,000 Lao Hmong have been smuggled into the northern Thai province of Loei in recent months.

Thai officials have begun interviewing the Hmong arrivals and say political refugees will be screened from those fleeing economic hardship. Suwit said earlier this month that "it is necessary for Thailand to arrest these illegal immigrants and send them back to their homeland the same way the United States is doing with Mexicans."

An estimated 680,000 Indochinese refugees have fled to Thailand since the 1975 communist victories in Vietnam, Cambodia and Laos. More than 100,000 remain in Thailand, waiting to be resettled, and another 260,000 Cambodian refugees live in border camps.

Thai officials often express concern that they will be stuck with hundreds of thousands of refugees because many other countries, including the United States, have been cutting down the number of refugees they accept for resettlement.Recent Vietnamese refugees interviewed at the Phanat Nikhom transit center said they fled to Thailand because a safe route across Cambodia had been developed.

Many of the refugees who fled Vietnam in the past dozen years left on small, poorly equipped boats that often broke down, capsized in storms or were attacked by pirates.