DETAILS are still scant on the substance of the accusations against Hong Kong-based journalist Xi Yang, detained by China yesterday on a charge of ''espionage regarding state secrets on banking''. It is too early to make any comparison with the case of another Hong Kong reporter, Leung Wai-man, who was arrested earlier this year and deported from China. It is not clear if the alleged secrets involved are more sensitive than the speech by Communist Party leader Jiang Zemin, which Leung was accused of paying for. Nor is it clear whether China believes money changed hands in the present case, although at this point there is no indication that any had. Nevertheless, the concerns raised by the Leung affair will resurface as a result of yesterday's events. The most worrying aspect is China's intolerance of journalistic activities and its extraordinarily broad definition of a state secret. No Hong Kong reporter working in Beijing can feel confident his next assignment will not result in arrest. Secondly, the punishments meted out to Leung's contacts in Beijing were alarmingly severe. An editor of Xinhua (the New China News Agency) was sentenced to life imprisonment and another reporter to six years. Xi, a recent immigrant from China, and not yet a Hong Kong permanent resident, may well find himself treated as if he were a mainland resident. He will be lucky if he is deported. Whatever the allegations against Xi, the Hong Kong Journalists' Association, Ming Pao and, above all, the Hong Kong Government should be pulling out all the stops to lobby on his behalf. A journalist's job is to search out information, although not to corrupt his sources by paying for it. He must not be subjected to arbitrary imprisonment by the state for doing his job.