Article 22(4) of the Basic Law of the Hong Kong Special Administrative Region provides that:
"For entry into the Hong Kong Special Administrative Region, people from other parts of China must apply for approval. Among them, the number of persons who enter the Region for the purpose of settlement shall be determined by the competent authorities of the Central People's Government after consulting the government of the Region."
For detailed information, please refer to the following link of the Immigration Department of Hong Kong:
Arrangement for entry to Hong Kong from the Mainland