THE continuing influx of illegal immigrants from China remained a big problem for police last year, with official records showing a 40 per cent increase. The Commissioner of Police, Li Kwan-ha, highlighted the problem in his annual report for 1992, which was released yesterday. ''In respect of illegal immigration, the huge influx of Vietnamese migrants over the last few years ceased abruptly following implementation of the Orderly Repatriation Scheme and the Voluntary Repatriation Programme. Unfortunately, in its place came theworrying trend of ever increasing numbers of illegal immigrants from China,'' he said. More than 35,600 IIs were arrested last year compared to 25,000 in 1991. The increase also prompted the deployment of more than 400 officers, concentrating on border patrol and daily searches. Last year, half of the arrivals came by sea, and the other half by land - a different pattern compared with the previous year when most travelled t o Hong Kong by boats.