CAPE TOWN, SOUTH AFRICA, JUNE 6 -- In the first electoral test of white opinion since the South African government released black leader Nelson Mandela and legalized the African National Congress last February, the far-right Conservative Party showed today that it is wooing significant support away from the ruling National Party of President Frederik W. de Klerk.

The Conservatives, led by Andries Treurnicht, more than doubled their vote and came within 547 votes of winning the hitherto safe government seat of Umlazi in Natal Province in a special parliamentary election. Treurnicht vehemently opposes de Klerk's attempts to reform the apartheid system and negotiate a new constitution with black nationalists.

At the last general election, in September 1989, the National Party polled 6,149 votes, winning the seat with a comfortable majority of 2,835 over the liberal Democratic Party, with the Conservatives runing in third place with 2,429 votes.

Today the National Party polled 5,762 votes to the Conservatives' 5,215, while the Democrats ran third with 982 votes.

The result indicates, analysts said tonight, that large numbers of Dutch-descended Afrikaners, traditionally the core of the governing party's support, are rejecting de Klerk's reforms and joining Treurnicht's far-right movement. At the same time, many white liberals, drawn largely from the country's British-descended minority who constitute 40 percent of the white community, appear to be abandoning the Democratic Party to save the government from defeat by the hard-liners.

However, the extent of the shift does not yet pose an electoral threat to de Klerk. He still has a two-thirds majority in a white House of Assembly of 177 legislators and does not have to face another general election until 1994.

Meanwhile, de Klerk is expected to announce more changes, including a lifting of the country's four-year state of emergency, when he addresses a joint session of the racially segregated three-chamber Parliament Thursday. Black nationalists have demanded the lifting of the emergency as one of their major conditions for entering into negotiations with the government.