The presiding magistrate at the inquest into the death of jailed black nationalist Steve Biko rejected a move by Biko's lawyer today to extend the scope of the injury into allegations that police had covered up the cause of Biko's death.

Meanwhile, it was reported that another jailed black, Bonaventura, S. Malza, 18, had died while in detention in Krugersdorp, about 30 miles west of Johannesburg. Malaza's relatives told the Rand Daily Mail that police had told them the youth hanged himself in his cell.

Malaza's brother said the high school student was detained July 1 by security police.He was the 21st person to die in custody since March 1976 - a death rate that has led to mounting criticism of South Africa.

At the inquest, Magistrate M. J. Prins refused to admit as evidence a file of statements by Justice Minister Jimmy Kruger implying that Biko had died of a hunger strike. An autopsy later showed that he died of head injuries.

The Biko family's lawyer, Sydney Kentridge, apparently had hoped to use the Kreger file in building a case that there had been a police cover up.

In another setback for Kentridge, Prins permitted a lawyer for the security police to introduce statements allegedly made by detained friends of Biko implicating him in subversive activities. Kentridge said the statements were not admissible because the authors are jailed under the Terrorism Act and cannot appear in court to authenticate them.

In rejecting the file of statements by Justice Minister Kruger, Prins said it was "hearsay to the third degree that is so far remote . . . with so many possibilites for misunderstandings, that it would be utterly dangerous to accept that as evidence."

By "remote," Prins meant that information about Biko's death passed through many people in the police chain of command before it got to Kruger.

Kentridge countered that the statements offered as evidence by the police lawyer were hearsay.

Prins added that the Kruger file would not be relevant to the main goal of the inquest, which is to establish the cause of Biko's death Sept. 12. The magistrate said Kentridge had other means to determine the credibility of police witnesses.

Kentridge had argued yesterday that unraveling the hunger strike cover story would go a long way toward finding out what really happened to Biko in jail, because whoever planted the story probably "had something very serious to hide."

The counsel for the police, PR. van Rooyen told Kentridge he would allow the high security police officials who gave information to Kruger to be available for a private consultation with Kentridge.

In the second ruling, Prins allowed Van Rooyen to introduce statements allegedly made by Biko's friends, which police have testified caused Biko to go into a rage, ending up in a scuffle with five policemen during which police suggest Biko may have received the head injuries that killed him.

Police said the scuffle occurred five days before Biko died.

Since the alleged authors of these statements, Patrict Titi and Peter Jones, are detained under the Terrorism Act, they cannot appear in court to verify that the statements are theirs, that they were made voluntarily or that were in the hands of the police before Biko's death.

Declaring that the issue was not the truth of the statements, but rather that they existed, Prins allowed them to be introduced as evidence. No attempt was made to put on the record what the prisoners said, and the judge forbade publication of the statements.

On Tuesday, Van Rooyen attempted to introduce similar statements that police said were shown to Biko. They were thrown out when Kentridge pointed out that they were dated after Biko's death. Today, the police produced different handwritten notes, dated before Biko's death, which they say are the ones shown him.

In addition, although the police have testified that it was these statements by Biko's friends that provoked the suffle Sept. 7, one police officer testified today that the documents were shown to Biko on Sept. 6.

Two international observers, Sir David Napley, for the Law Society of England, and University of Pennsylvania Law School dean, Louis Pollak, have arrived to observe the inquest, Pollak is observing for the Lawyers Committee for Civil Rights under Law, the American Bar Association and the Bar Association of the City of New York.