Police in the northwestern city of Yulin are tightening the hukou system to prevent people from having multiple household registrations, a senior official said on Thursday.

Hukou is the Chinese term for the country's permanent household registration system and is the basis for issuing personal ID cards.

Under Chinese law, a person can have only one hukou.

However, former bank official Gong Aiai was detained in February in Yulin, Shaanxi province, for having multiple hukou that she obtained by forging official documents and seals.

With multiple hukou, the 49-year-old, then vice-president of Shenmu Rural Commercial Bank, was able to buy 41 apartments in Beijing with a combined area of 9,667 square meters, police in the capital said.

Investigators found Gong also had at least two apartments in Xi'an, Shaanxi province, and two in Shenmu, a county under the jurisdiction of Yulin.

The case attracted widespread attention and Gong was given the nickname "House Sister" for owning such a large number of properties.

Gong is still under investigation and her case has not yet been transferred to prosecutors, Lu Zhiyuan, mayor of Yulin and a deputy to the National People's Congress, said on Thursday during the annual meeting of China's top legislature in Beijing.

Lu said after the case was exposed, Yulin police started a campaign to clean up the city's hukou system. He said the campaign is ongoing but did not disclose how many violators had been found.

According to media reports, it is not rare for rich people in Yulin to have multiple hukou or ID cards so they can buy property with different accounts.

Jiang Zelin, deputy governor of Shaanxi and an NPC deputy, said on Thursday the investigators are looking into Gong's fake documents and whether her assets were acquired legally.

"We'll let the public know the results once the probe is over, and any illegal activities will be punished," he said.

Shenmu Rural Commercial Bank is a business, and Gong, as a long-term employee, was not a civil servant, Xinhua News Agency reported.

Responding to media questions about the hukou system shake-up, Shaanxi Governor Lou Qinjian called on Tuesday for a unified information network of identification data, including hukou and ID cards.

"On one hand, the government should manage the hukou system more effectively," said Lou, also a NPC deputy. "On the other hand, hukou management relies on the construction of an integrated networking system."

National political advisers in Beijing for the annual session of the top political advisory body, the National Committee of the Chinese People's Political Consultative Conference, have called for harsher penalties for a small number of people who make use of their power or money to illegally hold a large number of properties.

"Such activities help push up the housing prices, triggering public complaints," Cao Dewang, chairman of Fuyao Glass Industry Group and a CPPCC member, was quoted as saying on Wednesday in a report on the People's Daily website.

"They also keep a large amount of land and resources from market circulation, which directly or indirectly harms the national economy," he added.