PUBLIC estate tenants will no longer be able to rely on caretakers to fix water or electricity supplies after midnight, when about 700 staff start a work-to-rule campaign from tomorrow. Tenants last night condemned the move as an ''irresponsible action by lazy staff'', and Housing Department officials warned the strikers they would have wages deducted. About 700 estate assistants are expected to join the protest, which is prompted by the department's refusal to accept their demand for an immediate pay rise by two points on the pay scale - up to $1,000 a month. Their salaries range from about $7,000 for a junior estate assistant to $12,000 for a chief estate assistant. A protest organiser, Tang Fook-cheung, said: ''Very often we have to work overtime after midnight. We are not supposed to work after midnight and our overtime work is not paid. ''An $8,000 monthly wage is not proportional to our workload. We asked for a review three years ago but nothing has been done. It is time for us to act, to show our strength.'' Mr Tang, whose group claims to represent 700 of the 2,000 estate assistants, warned that they would step up action if their demands were not met. But the vice-chairman of the Hong Kong Association for Democracy and People's Livelihood, Leung Kwong-cheong, expected the action to have minimal impact on daily estate management. ''A common complaint by tenants is that many caretakers are too lazy to fix things for them, not to mention in the early hours. In most cases tenants have to report problems to housing managers in person,'' he said. The department's principal executive officer, Ahmet Gafoor, said he was surprised by the caretakers' action because he had agreed at a meeting with them last week to relay their demands to the Civil Service Branch. Mr Gafoor said they had had a special wage increase in 1991 and many of their duties had since been transferred to housing officers. He warned that they might risk having wages deducted if they refused to perform duties during office hours.