OTTAWA - Canada's environment commissioner says the Conservative government's estimated cuts to greenhouse gases are off by at least half.

A large portion of the government's anticipated cuts to greenhouse gases won't actually occur, Scott Vaughan told a parliamentary committee on Tuesday.

"We probably have a ballpark of at least half or perhaps a little more," Vaughan said when asked by how much the Tories' projections are off.

"We didn't want to get into specific numbers, but that would be sort of a general estimate."

Vaughan appeared before the House of Commons environment committee to talk about his spring audit, tabled earlier this month.

The report found the Conservatives have overstated projected cuts to greenhouse gases in their green plan -- but it didn't say by how much.

New Democrat MP Nathan Cullen -- subbing for another NDP member of the committee -- likened the Tories' over-estimation on greenhouse-gas cuts to their under-estimation of this year's deficit.

On Monday, Finance Minister Jim Flaherty said the financial shortfall, forecast in January at $34 billion for 2009-2010, will be much higher.

"It's almost like the deficit numbers this government runs. They seem to miss by a lot," Cullen said of the greenhouse-gas numbers.

Environment Minister Jim Prentice was in Paris for the Major Economies Forum and unavailable to comment. A spokesman from his office did not directly address Vaughan's remarks to the committee.

"We are introducing tough new regulations to limit greenhouse gas emissions from the automotive sector, investing in low carbon technologies like carbon capture and storage and working with the U.S to establish a clean energy dialogue," said Frederic Baril in an email.

"We are also continuing to fine-tune our domestic policies and intend to put the regulatory framework to reduce greenhouse gas emissions into law in the coming year."

But the environment commissioner's audit also found that Environment Canada has no way of tracking how effective those specific policies are in curbing emissions.

The department says it's too costly and technologically cumbersome to track how much particular policies lower greenhouse gases.

The audit looked at both the federal government's compliance with the Kyoto Protocol -- an international treaty ratified by the previous Liberal government that the Conservatives have eschewed -- and the Tories' own climate-change plan.

Under Kyoto, Canada agreed to lower its greenhouse gases six per cent below 1990 levels between 2008 and 2012. The Conservatives say those targets are unattainable after years of inaction by their predecessors.

The Tories instead have adopted the more modest goal of reducing greenhouse gases 20 per cent from 2006 levels by 2020.

Environmental groups and opposition parties have criticized the Conservative plan for using 2006 as its baseline year, when emissions were higher than 1990 levels.

Canada emitted 721 megatonnes of carbon dioxide equivalent in 2006, compared to 592 megatonnes in 1990.

To reach its target, the government has proposed regulatory measures, a cap-and-trade scheme, investments in green-technology funds and credits for companies that reduced their emissions early.