Hong Kong is a city of tourism, with China providing more than half of the visitors each month. However some of these Mainland tourists are not coming to look at the scenery. From the Financial Times:

...one of the dilemmas posed by Hong Kong and southern China's increasingly close relationship: how to police a porous border. Beijing's decision to relax visa restrictions on mainland travellers to Hong Kong, which has brought about 2m Chinese visitors into the city since last summer, and growing business ties with southern China's Pearl River Delta, are putting new pressure on police, courts and the criminal justice system.

"For criminals, there are no boundaries," says Dick Lee, Hong Kong's police commissioner. "Wherever there's money to be made, they will go."...

Mainland Chinese immigrants steal wild turtles and herbs, honey and "fragrant trees" from parks in Sai Kung, eastern Hong Kong, according to Mark Johnson, divisional commander for the police in that area. So many drain covers have been stolen from one Sai Kung road that workers have stopped replacing them. They are sold to scrap metal dealers on the mainland...

Hong Kong police have no jurisdiction in mainland China. Instead they must work through public security bureaux in individual provinces, adding a time-consuming layer of bureaucracy.

"They are constrained by process, as all police officers are when they get out of their own zone," says Steve Vickers, a retired Hong Kong policeman who now runs the consultancy International Risk...

Not everyone slips through the net. Mainlanders make up a third of the city's prison population - 4,400 out of 13,300 inmates, according to the government.