Clark's calls came after Hazelwood power station chief Graeme York received a hand-delivered warning in the mailbox of his Melbourne home, allegedly from the radical group Earth Liberation Front (ELF).

But prominent Melbourne barrister Brian Walters SC told Lawyers Weekly that he would be very opposed to strengthening the current laws.

"We already have undercover police infiltrating environment groups and undermining their work for a safer planet and we don't want any more powers [given] to [police] of this nature," he said.

"There has been a lot of experience in the past in relation to claims of eco-terrorism in Australia and they have all been hoaxes - every one of them and there have been many of them - and we should approach this with great circumspection."

Walters said examples included the 1998 federal election in Tasmania when a hoax bomb was placed on a train carrying native forest logs.

"It got front-page coverage in Tasmania in the papers a day before the election and the day of the election that these greenies were stooping to placing bombs. [But] subsequent police investigations found that it was placed by anti-environmentalists, not by environmentalists, but it was too late then because the Greens candidate for the Senate had failed to win in that election because of that bad publicity," he said.

"I've been involved in many hearings getting material about this and there has just been a legion of allegations that have proven to be false later."

The letter to Hazelwood's chief declared that he was responsible for the "dirtiest power station in Australia and the most polluting in the industrialised world".

"We hold you personally accountable for this assault against our earth. The irreplaceable and precious eco-systems of this earth are worth much more than your manicured lawn, expensive car and opulent suburban home," the letter said.

"Your property will not remain safe so long as Hazelwood continues to pollute at such an inexcusable level, swallow millions of litres of fresh water every hour and cough out hydrochloric and nitrogen acids in return."

The letter was being investigated by special intelligence detectives, said Victorian Police Commissioner Simon Overland.

The ELF has not been prominent in Australia, but a US spokesman for the group told AAP that York "should consider himself lucky" to have received a hand-delivered warning. The group has been responsible for a series of fire-bomb attacks on industrial property and equipment in Europe and the US.

State Energy Minister Peter Batchelor said he would ask the Victorian Government to make sure adequate laws existed to deal with "these type of activities".

- Sarah Sharples