ONLY one in every 5,000 mainlanders stayed an extra day while passing through Hongkong last year, said the Secretary for Security, Alistair Asprey. Although mainlanders in transit through Hongkong were allowed to stay for seven days without a visa, he said the Immigration Department's computer records would expose anyone who stayed too long. ''We will maintain effective pre-entry and post-entry controls to prevent overstaying,'' he said. Mr Asprey pointed out that visitors who took jobs in the territory were liable to prosecution and, if convicted, could be fined up to $5,000 and imprisoned for two years. Around 1,000 foreign hawkers were arrested between 1990 and May 1993. Mr Asprey added that several checks would be taken to expose any attempt to obtain right of abode in Hongkong after 1997 by fraudulent means for mainland children with at least one parent who was a permanent resident here.