Families of the five have sought help from the Society for Community Organisation and Isinolaw Research Centre, which offers free legal assistance to people facing a detention ordeal.

The Security Bureau has admitted it is following up on three cases brought to its notice by family members that did not pass through the notification system. It is not clear if these three cases involve those being handled by the two help groups.

All five cases being handled by the help groups fulfil the criteria for the notification mechanism.

One of them, a man in his late 40s, was recently transferred to a Guangzhou detention centre after being confined nearby in a Huadu city jail since early last month by Customs authorities over an alleged tax problem.

The Society for Community Organisation, which has been following up the case, said the man's wife learned about the news only when SAR police handed her a residential surveillance certificate issued by mainland officials more than a month after his arrest.

Human rights activist and society chairman Ho Hei-wah said: 'The notification system exists for nothing. It cannot serve the purpose of helping monitor whether detainees' rights are being violated. The Security Bureau should take each case seriously in updating them and should press hard to chase the unreported cases.'

Founder and director of Isinolaw Research Centre Priscilla Leung Mei-fun said: 'The mainland Government should step up publicity to educate its district officials about the new system, especially in the remote areas.'

Dr Leung, also associate law professor at City University, said many of the cases being followed by her centre had well exceeded the initial statutory detention period of 37 days.

The Security Bureau refused to comment on whether there was a lack of co-operation from mainland authorities.

Besides following up the three unreported cases, the bureau had also managed earlier this year to track down another three based on family members' information, a bureau spokeswoman said yesterday.

The notification system covers people being arrested, detained, those who are under residential surveillance, on bail or people being interrogated by the judiciary after failing to report as required.

The reciprocal arrangement was reached after 18 months of negotiation between the SAR and the mainland amid intense press reports about the prevalence of Hong Kong people being locked up for prolonged periods for commercial reasons.

Mainland public security bureaus and Customs are required to report to Beijing's Public Security Ministry once an SAR person is taken into their custody.

In turn, the ministry will inform the SAR police, who will notify family members immediately.

Hong Kong has so far reported 518 cases to Beijing, including 325 related to immigration violations.

At present, 68 SAR people are being held, mostly in Shenzhen, with others in Zhongshan, Dongguan, Liaoning, Hunan, Shanghai and Zhejiang, according to the notification system. Most are being confined on fraud and smuggling charges.

Legislators have demanded the system be expanded to cover mainland authorities other than the public security bureaus and Customs after the arrest of locally based scholar Li Shaomin.

American-Chinese Dr Li, 45, an associate professor at City University, is said to have been picked up by Shenzhen officials from the Ministry of State Security for undisclosed reasons.

The notification system does not cover people arrested by Ministry of State Security officials.