Barbara Hogan, a 30-year-old white woman, was convicted of treason in the provincial Supreme Court here today after admitting she helped the outlawed African National Congress.

The effect of the treason conviction, the first of a white woman in South African history, is to escalate the penalty for aiding the congress, the main African nationalist movement in this country, from a maximum 10 years' imprisonment to possible capital punishment.

Hogan is unlikely to be sent to the gallows when Judge H. P. Van Dyk passes sentence. In his judgment today he said he regarded her as guilty of treason in a technical sense only, because the African National Congress is committed to a guerrilla campaign aimed at overthrowing white rule.

Nevertheless, lawyers connected with the trial pointed out that the judgment establishes case law transforming support for objectives the African nationalist movement from a comparatively modest offense to a capital one.

In practice most first offenders who are not significant leaders of the banned party have been jailed for two to three years. Lawyers say this is likely to increase considerably now that the offense of aiding the group is considered treason, and that in some cases the death sentence is likely to be imposed.

The judgment is seen as reflecting government concern at growing support for the congress.

According to a CIA report leaked to South African journalists in Washington this week by TransAfrica, a black American lobby group on African affairs, this support has grown significantly since police inflicted heavy casualties while suppressing riots in the black township of Soweto in 1976.

The military wing of the congress has the capacity to inflict increasing damage on installations and is under pressure from its young post-1976 recruits to switch to civilian targets, according to the report leaked by TransAfrica.

Hogan, who was detained under South Africa's stringent security laws in September, pleaded guilty to joining the congress and to carrying out some of its instructions, including one to furnish its leaders in exile with documents on the emerging black labor union movement.

She pleaded not guilty to the treason charge, claiming she had not been involved in the organization's military wing nor any aspect of its guerrilla campaign.

The judge rejected Hogan's claim that she had played a nonviolent role within the congress, saying she must have been aware that assisting it would help it achieve its revolutionary aims.