Six weeks after the South African government won widespread praise for announcing a suspension of forced removals of black communities, a court judgment delivered in Pretoria has shown those communities are still vulnerable.

Government statements have made clear that, despite the moratorium, these communities still can be uprooted in a "negotiated" removal if their chiefs agree, and last week's court judgment confirmed that the white-minority government can appoint its own choice as chief of an African tribe.

The case was brought by a committee of the little Kwangema community of eastern Transvaal Province, whose land was given to them 125 years ago by the ancestors of the white Afrikaner minority.

They have put up a spirited resistance to the government's plans to move them off their ancestral farming land to a packed resettlement camp 100 miles away, in accordance with South Africa's apartheid policy, which seeks to divide the population into ethnic compartments.

The government announced the suspension of all forced removals Feb. 1 while it reviewed the resettlement program. The Reagan administration and other western governments welcomed what seemed to be an important change in policy.

However, the minister in charge of black affairs, Gerrit N. Viljoen, said at the time that "negotiated" removals would still go ahead, and he indicated that this meant obtaining the approval of a recognized tribal chief.

The point Viljoen made to illustrate this was that he regarded the removal of Magopa village last year -- when armed police threw a cordon around the community in the middle of the night, declared it an "operation area" and forced the villagers into trucks -- as a "negotiated removal" because the man the government recognized as the community's chief had given his consent.

The people of Kwangema fear that the same may happen to them.

A discredited figure in Kwangema, Cuthbert Ngema, who already had indicated that he was prepared to agree to the removal, was named acting chief last November.

Cuthbert Ngema was chosen head of the community of 5,000 at a private meeting of eight relatives held in his own home while the rest of the community was gathered elsewhere. The white administration, two of whose officials had attended the private meeting, recognized the election and named him acting chief soon afterward.

His position means that he can demand the "loyalty, respect and obedience" of the community, and he can banish or arrest any members failing to render it.

An elected committee, headed by another family member, Moses Ngema, started a court action to have Cuthbert Ngema's appointment declared invalid, but because the case could take more than a year to be heard -- during which time the acting chief might conclude the fateful "negotiation" -- he also sought an urgent injunction preventing him from occupying the post in the meantime.

That was the case on which the court ruled last week.

Noting that the community had not had a chief for 30 years, the committee's lawyers said they were convinced the government had appointed Cuthbert Ngema to this position "to facilitate the removal in the face of opposition from Moses Ngema and his committee."

The way Cuthbert Ngema became acting chief was not at issue. This was because of a law in force for more than a half century empowering the minister in charge of black affairs, or a designated official, to appoint the acting chief of an African tribe.

Because of this, the Ngema committee's lawyers had to base their case not on the manner of the appointment, but on the technicality of whether the little community constituted a "tribe."

Lengthy affidavits from social anthropologists were submitted by both sides to argue this point, and the case turned on that issue rather than on whom the community regarded as its real leaders and whether the white administration was taking advantage of it.

Judge T.T. Spoelstra tried to resolve the issue by suggesting that the community hold an election, but the government's lawyers refused.

Delivering his judgment, Spoelstra quoted legal precedents to show that the community should be regarded as a "tribe."

The judge pointed out to the unhappy committee that Cuthbert Ngema, in papers before the court, had said that he would consult the community before agreeing to a removal. But, as lawyers pointed out afterward, that agreement has no force in law and can be ignored by both Cuthbert Ngema and the white administration.