ONLY 12.4 per cent of 6,440 mainlanders who overstayed on their two-way permits have been prosecuted since policy was changed last April. Figures provided by the Immigration Department show that from April to November last year 796 overstayers were prosecuted and convicted. A department spokesman said most had been fined between $500 and $2,000 but a few had been jailed for up to three months. Although there are provisions to prosecute all foreign visitors who overstay their visas, no mainlanders had been prosecuted before the new policy came into effect last April. The Government dropped its lenient approach after a record 7.6 per cent, or 22,566, of 298,500 mainland visitors overstayed in 1991. The spokesman said the department did not regard the prosecution rate as low because it charged only those who had deliberately overstayed or who could not provide a reasonable explanation for doing so. He said most mainland visitors overstayed only for a short period and did so inadvertently. Two-way permits for mainlanders on group tours are valid for one week. Permits for individuals are valid for up to three months. Since the new policy was introduced the monthly tally fell from 2,097 in February - when the policy was announced - to 743 in April and 410 in May. However, the falling trend reversed with the October and November figures standing at 956 and 1,116 respectively. He said the number of overstayers grew in the second half of last year probably because they wanted to stay in the territory to celebrate a number of festivals, including the winter solstice. The situation would be closely monitored through more random identity checks in restaurants and factories.