ACT MP David Garrett says his party will push for the 'three strikes and you're out' legislation to be toughened before it goes back to the House.
The bill, which is ACT Party policy, was supported to select committee by the National Party as part of a confidence and supply deal.
But Mr Garrett says the bill has been watered down, with offenders having to not only be convicted of a serious offence, but also sentenced to five years or more in prison.
He says that when he agreed to the five-year provision he had no idea it would weaken the legislation so much.
Prime Minister John Key says the Government has not tried to undermine the legislation.