At least 400 tonnes of rocks were sold to beautify the Civil Engineering Department's artificial lake project near the Disney site in Penny's Bay.

If the Lands Department pushes for prosecution, Mr Law could face two charges under sections 7 and 8 of the Land (Miscellaneous Provisions) Ordinance, which governs illegal activities on government land. The maximum penalty for each charge is a $5,000 fine and six months' imprisonment.

But Man Chi-sum, the chief executive officer of Green Power, said the penalties were too light to deter future acts of eco-vandalism.

'Has anyone been jailed for environmental offences before? The offender might just be fined and walk away from court without repenting. Tougher penalties are now urgently needed.'

A source familiar with the incident said: 'This is a test of the district lands office's commitment to protecting government land. A jail term, even of only one day, would be appropriate because of the seriousness of the offence.'

The $5,000 fines under the lands law are far less than the $200,000 fine for dumping on government land or at sea. Dr Man called on officials to review the existing legal framework to protect valuable sites because 'the government is failing to protect its land'.

He is calling for a river protection law to safeguard other streams in Hong Kong and is trying to arrange a meeting with Secretary for the Environment, Transport and Works Sarah Liao Sau-tung to discuss this.

A spokeswoman for the Housing, Planning and Lands Bureau said yesterday that charges could still be laid after the river was restored. The Lands Department will hold a meeting with Mr Law to discuss the river restoration proposal within a few days. Green Power is urging the department to bring in experts and environmentalists for the works.

The department said it had asked police to investigate whether there was any element of crime involved, but police said they were still studying the matter. The Home Affairs Bureau, which maintains daily liaison with rural committees, said Mr Law had not informed it of the excavation because there was no law regulating the operation of the committee.

Villager Wong Yat-wah, the contractor who performed the river excavation, said the work was genuinely carried out for flood prevention. 'For thousands of years in Chinese society, it has been the responsibility of rural committees or village heads to do good deeds for their villagers.'

He denied having anything to do with the sale of the rocks and refused to disclose more details. The boulders are now being stored on private land owned by Mr Wong.

A spokesman for the Drainage Services Department said it had never received any reports of flooding near the Tung Chung river.