A top official in the Hong Kong government has estimated that Vietnam will earn $3 billion in foreign exchange through continued massive expulsion of refugees to Southeat Asia and the United States.

A trial here is revealing details of the organized Vietnamese effort to profit from the refugee trade, which Southeast Asian leaders are beginning to denounce as a deliberate effort to strain their resources and improve Hanoi's finances.

"We know that the Vietnamese government regards this trade in human lives as a major source of foreign exchange," said Information Secretary David Ford in a speech here Thursday. "Indeed, it is now said to have overtaken their largest export earner, their coal industry."

Ford provided no figures to back up his statements, but it appeared to be based on estimates that so far 300,000 Vietnamese residents have left the country on boats after paying fees of $350 to 3,000 each. A spokesman for Ford's office said he also was calculating the number of people who would leave Vietnam in the future and the amount of remittances they could be expected to send back to relatives in Vietnam.

A government prosecutor has presented evidence at trial of the crew of a freighter that brought in 3,318 refugees showing each adult paid about $4,000 in gold and each child under 16 about $350. The prosecutor, Peter Duncan, said the freighter Huey Fong picked up people brought by Vietnamese authorities to Tau Tun harbor south of Ho Chi Minh City (Saigon). The ship telegraphed Hong Kong saying the refugees were rescued off boats foundering far off shore.

Four Huey Fong passengers and three officers are being tried in Victoria district for conspiracy to defraud the Hong Kong government. Four other officers already have pleaded guilty in helping create the Christmas season drama, in which the freighter full of children was kept outside Hong Kong water for a month before finally allowed to dock.

The crew and the refugees, allegedly carefully drilled during the voyage, said at first they had been picked up at sea. This story began to unravel when investigators found more than $1 million in gold hidden in the ship's hold.

Duncan said the Huey Fong spent two days loading the refugees at Tau Tun after each person registered with Vietnamese officials and paid a fee. The money in the hold, only a small portion of what was collected from the refugees, allegedly was to pay for the hire of the ship and crew.

The scheme was organized through a series of cables between Hong Kong residents and their friends and relatives in Vietnam, Duncan said.

Refugees have arrived in a massive surge here in recent weeks in smaller boats, since the freighter scheme seemed to have failed. They say in interviews that they also arranged through Vietnamese officials to have the boats purchased and the sailing date set. Some said they were given the choice of leaving by boat or being placed in concentration camps.

Ford, the information secretary, accepted estimates made by refugees arriving here that at least half of the people setting out from Vietnam, usually on boats much smaller than the Huey Fong, have been lost at sea. Only about 150,000 have reached Malaysia, Hong Kong or other scattered shores.

[In Manila, Reuter reported, several of 2,318 refugees to reach the Philippines aboard the freighter Tung An told an official inquiry they paid in gold bars or dollars for passage. The ship's Taiwanese master, Dah Shing Sheu, said he stopped to rescue them from small boats off Vietnam but the refugees said they boarded the freighter at Tu Doc on the Vietnamese Coast.]

At least 80 percent of the 46,000 boat refugees jammed into Hong Kong docks and barracks are ethnic Chinese, victims of official persecution caused by their relative wealth in Vietnamese society and their potential ties to Peking. Hanoi's archenemy. Ford said Hanoi was deliberately "condemning to death by drowning over half of a million of its own citizens simply because they happen to be Chinese."

Vietnam was estimated to have about 1.4 to 1.8 million ethnic Chinese 1n 1977, when Hanoi began to expel refugees in large numbers by boat and across the land border with China. Ethnic Chinese refugees reaching here estimate that at least a million would try to leave if given the opportunity.

The United States has pledged to take about 7,000 Indochinese refugees a month, but boat people are leaving here for the United States at a rate of about 400 to 500 a month. Ford said the nations of the World "must come together intent upon the task of bringing home to the Vietnam government the full horror of their policy."