SYDNEY — AUSTRALIA pulled its troops out of Vietnam in 1973. But Australian business leaders are now mounting an invasion of its former enemy through a variety of trade and business ventures.BHP Petroleum, a subsidiary of BHP Company Limited, Australia's largest company, is prospecting for oil off Vietnam's shores; Telecom, the Australian telephone company, is trying to sell telecommunications services; and entrepreneurs are developing tourism packages. The actual amount of trade so far is relatively modest. According to the Department of Foreign Affairs and Trade, in 1990 Australia exported $81 million (Australian; US$65 million) to Vietnam and imported $18.7 million. However, Australian exports were swollen last year by the sale of a floating hotel to Vietnam for $53.5 million. In an effort to expand trade, Neal Blewett, minister for trade and overseas development, is leading a delegation composed of 21 entrepreneurs, bankers, lawyers, and corporate managers to Hanoi and Ho Chi Minh City in late November. The Australian government's involvement in encouraging the trade is likely to accelerate now that the Cambodian peace agreement has been signed. Although private companies have been trying to do business with Vietnam since 1987, direct government involvement was delayed. "We wanted full Vietnamese cooperation in the settlement process," says a foreign affairs official. Australia is not alone in its interest. Singapore recently lifted its ban on investment in Vietnam. And the United States is inching closer to reestablishing full ties with Vietnam. In October, Secretary of State James Baker III said the scope and pace of restoring relations will be governed by the degree to which Vietnam cooperates on resolving prisoner-of-war and missing-in-action issues. Australia has accounted for all of its soldiers who fought in the war, and therefore has no political obstacles that could hinder its commercial relationships. Over the past two years it has signed a number of trade and investment protection agreements with Vietnam. The latest trade mission allowed companies to meet with the top officials in the Vietnamese bureaucracy. Although Vietnam is one of the world's poorest nations, it also has its attractions. Oil, for example. BHP Petroleum is currently evaluating some seismic tests made off the Vietnamese coast. If the prospects look good, drilling will start in the middle of next year. BHP and other major companies in the area, such as BP and Shell, are also looking at some leases that were formerly under license to Vietsov, a Soviet drilling venture. According to press reports, the Soviets did not have the funds to explore for oil and withdrew from the Con Son basin in the South China Sea. "As yet it has unproven potential, but the early indications are that it is oil prone," says Jeff Mitchell, an official of BHP Exploration in Melbourne. Vietnam currently produces about 50,000 barrels of oil per day. The oil, called bach ho, is a good quality waxy sweet crude oil, comparable to Arab light. Some other entrepreneurs think there's potential in tourism. A company called Direct Flights is now running two charters a month between Sydney and Vietnam. The flights cost $1,390 roundtrip and are almost fully booked. A Vietnamese operator, Pacific Airlines, applied for four charter flights to Australia in October, but had trouble obtaining an airplane. Many companies that produce or lease airliners are based in the US and are forbidden by a US trade ban to deal with Vietnam. Melbourne-based Vietnam Ventures is planning a train trip from Hanoi to Ho Chi Minh City in April 1992. So far, 50 Australians have signed up for the $3,000 tour. The company is run by Dorothy Button, the wife of Sen. John Button, Australia's minister for industry and commerce. Since cash is scarce in Vietnam, many deals involve barter. For goods and services, Vietnam often pays with its major export commodities, including rice, coal, minerals, coffee, prawns, and fish. One interested buyer is Independent Seafoods, a Perth-based company, which pays for the fish in hard currency. Independent has also sold the Vietnamese several food factories for cash, including a slaughterhouse that produces meat that is consumed in the country or sold to the Soviet Union. "Vietnam needs a lot of help in rebuilding," says an Independent official who did not want to be named. "There's a lot of potential." Despite the potential, businessmen also say dealing with Vietnam can be frustrating. The government is still communist with an extensive bureaucracy that must approve almost all deals. "You need patience," says one businessman who has been doing business with Vietnam for 10 years.