Namibia is sliding into economic ruin as it waits for independence.

A secret government report here shows that if the rate of decline during the past eight years continues for another four, annual interest payments to South Africa will equal the territory's gross national product.

South Africa, which has contributed to the swelling debt by imposing an unwieldy segregationist constitution on the territory, is keeping the budget balanced with big injections of capital: $610 million--excluding defense expenditure--in this fiscal year.

This means that should the drawn-out negotiations produce independence, the incoming black government--likely to be formed by the South West Africa People's Organization or SWAPO--will find itself taking over a bankrupt country that will be as much of an economic hostage to South Africa as some of the tribal "homelands" the Pretoria government has established under its own segregationist system called apartheid.

According to a 16-page confidential report drawn up by the territory's finance department, Namibia's outstanding debt equals 130 percent of the current national income. The report warns that the debt "could amount to more than four times the expected annual income in the next four years."

By that time, the sum owed to South Africa in annual interest repayments will equal Namibia's gross national product, the report adds.

Not all of Namibia's woes are attributable to the control that South Africa continues to wield under a previous League of Nations mandate. Recession and the worst drought of the century have taken a heavy toll.

More than 35 percent of the territorial administration's revenues used to come from the huge De Beers diamond concession, which occupies 21,000 square miles on the Atlantic coast. The recession, which has weakened the demand for gems in the United States particularly, has knocked that back to 4 percent.

The recession has also brought the once lucrative karakul industry to its knees. Weak demand for karakul furs in West Germany, which buys 70 percent of the pelt shipments, sent the price plummeting from an average of $24 per pelt in 1976 to $9 last year.

That and the drought have reduced the number of karakul sheep in the territory from 3 million to an estimated 900,000, according to Fritz E. von Koehne, manager of the karakul board in Windhoek.

Drought has also devastated the meat industry. Cattle ranchers have been forced to slaughter more than 50 percent of their herds. The territory's biggest meat marketing organization, Damara Meat Packers, has announced that it will close one of its two slaughterhouses and lay off 400 workers in the next three months.

Fishing, once another mainstay of the economy, is in ruins. Although there are faint signs of recovery now, excessive quotas granted earlier by the South African government led to a plundering of the rich offshore waters. Tough restrictions had to be introduced in 1980, by which time an annual pilchard catch of more than 1 million tons had dwindled to 11,000 tons.

These economic setbacks are compounded by the constitutional system, whose waste, corruption and inefficiency are targets of an investigation by a commission headed by Judge P. W. Thirion.

The constitution divides government in Namibia into three "tiers." The first tier, or central government, deals with matters of common interest. It was controlled by Dirk F. Mudge's Democratic Turnhalle Alliance, but South Africa found the setup unsatisfactory and suspended it in January. Since then, the Pretoria government has run the first tier directly through its administrator general, Willem van Niekerk.

The second tier consists of 11 authorities set up to administer the affairs of the different ethnic groups. The system has led to a remarkable multiplicity of government departments for one of the world's most sparsely populated countries.

There are 11 departments of education, 11 of health, 11 of pensions, and so on, for a population of just over 1 million, or little more than half of Johannesburg's black township of Soweto.

The third tier encompasses local and municipal government, again separated on a racial basis.

Cynical Namibians have coined the phrase "one man, one government" to describe this system. According to the report, the system is hugely wasteful, eating up 75 percent of the national budget and employing 44,692 people.

"The only growth sector is government," Namibia's director of finance, Johan Jones, has been quoted as saying.

The unwieldy bureaucracy, tangled lines of authority among the maze of departments, and the poor educational base that has left thousands of posts filled by untrained people, have combined to cause large-scale inefficiency and corruption.

Kenneth Abrahams, editor of the authoritative Namibian Review, has noted that of the 400 Namibians who received their school certificates last year, only 40 were black. Of those, 12 qualified for university entrance.

"There is no attempt to provide a base of skills in preparation for independence," Abrahams said in an interview.

So far, the Thirion Commission has dealt with only two ethnic administrations, the Ovambos and the Damaras, but already its findings are causing an outcry.

The commission has found that officials built houses for themselves with unauthorized funds. One installed wall-to-wall carpeting "to make his wife happy." When the ethnic administration found out, it decided to cover up the matter by having all officials' homes carpeted.

A Portugese refugee from Angola named "Banana Joe" Alves was unaccountably paid $700,000 for an unknown quantity of gravel supposedly delivered to unknown points in the tribal area of Ovamboland, the commission found.

In another instance, a firm of architects was paid $1 million for work that was never done. In a random check, the commission found that 18 officials had signed for the salaries of 173 people. Another check showed that hundreds of people who did not exist were receiving social pensions.

Another commission headed by another judge has found the territory's health services so poor that it recommends they be taken away from the second tier authorities and be administered by the central government.