Information-sharing measures in proposed anti-terrorism legislation are so broadly worded they would allow the government to spy on its political foes, NDP Leader Tom Mulcair says.

Mulcair took exception Tuesday to the bill's mention of interference with infrastructure or economic stability as activity that undermines the security of Canada.

The wording is sufficiently vague to permit a Canadian Security Intelligence Service investigation of anyone who challenges the Conservatives' social, economic or environmental policies, the Opposition leader said during the daily question period.

"What's to stop this bill from being used to spy on the government's political enemies?"

Prime Minister Stephen Harper dismissed the suggestion, telling the House of Commons the NDP had entered the realm of conspiracy theory.

"That's what we've come to expect from the black helicopter fleet over there."

'New and astonishingly broad concept'

The bill introduced late last month would give CSIS power to disrupt suspected terror plots, thwart financial transactions and covertly interfere with radical websites.

The legislation would also relax the sharing of information about activity that undermines the security of Canada — "a new and astonishingly broad concept," law professors Craig Forcese and Kent Roach wrote in an analysis published Tuesday.

It comes close to authorizing a "total information awareness" approach to security and in that sense "we consider it a radical departure from conventional understandings of privacy," say the authors.

Even as it erodes privacy, the bill fails to learn from the lessons of two federal commissions of inquiry that documented the effects of uncontrolled information-sharing on Arab-Canadians, including Ottawa's Maher Arar, who were imprisoned and tortured in Syria, the national security experts say.

Government claims that existing watchdogs will provide a check against these powers are not convincing, Forcese and Roach conclude.

The bill says activity that undermines Canadian security does not include lawful advocacy, protest, dissent and artistic expression.

Environmentalists worried

Still, some environmentalists wonder if the new legislation will authorize CSIS to step up surveillance of activists.

Green party Leader Elizabeth May noted some demonstrations are not lawful but also do not involve violence -- for instance a rally to block an oil pipeline.

"Will non-violent, peaceful activities be exempted from this act?" she asked Tuesday in the House.

Harper said the bill was "designed to deal with the promotion and actual execution of terrorist activities, and not other lawful activities."

Keith Stewart, an energy campaigner for environmental group Greenpeace Canada, has difficulty accepting government assurances — particularly given the recent leak of an RCMP intelligence assessment entitled Criminal Threats to the Canadian Petroleum Industry.

The January 2014 assessment, initially obtained by Montreal's La Presse newspaper, says those within the movement willing to "go beyond peaceful actions primarily employ direct action tactics, such as civil disobedience, unlawful protests, break-and-entry, vandalism and sabotage."

The bill strengthens CSIS powers without an accompanying increase in oversight — a recipe for misuse, Stewart said.

He also worries about where the fruits of investigations will end up.

"I don't want tax dollars paying for Canadian spies to conduct what would otherwise be illegal surveillance on environmental groups and then passing this on to the oil industry."