The Government's changes to the Emissions Trading Scheme could cost $670 million more than estimated and will have little impact on reducing greenhouse gas emissions, the parliamentary commissioner for the environment, Jan Wright, will tell MPs today.

She says the changes - which include locking in large subsidies to polluters - will stop New Zealand reaching the Government's own target of halving greenhouse gas emissions by 2050.

Previous changes weakened the ETS, but the latest changes "render it almost toothless", Dr Wright says.

Cabinet papers indicated the Government's motive for changing the scheme again was to provide more flexibility and mitigate short-term costs for business while encouraging a low-carbon economy. But Dr Wright will tell Parliament's finance and expenditure committee today there will be little direct cost for emitting carbon dioxide, even in the long term.

"Without a direct cost, there will be no transition to a low carbon economy, smooth or otherwise. Major polluters will still be paying for just 5 per cent of their emissions in 2050."

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