ACT's three strikes policy, where offenders would be imprisoned for life after committing three serious crimes, is included in a government bill being considered by a select committee.
The inclusion was part of a post election deal with the National Party but now ACT is saying it may not support it.
ACT has always been unhappy about setting a bar of sentences of five years or more to count as a strike offence.
Yesterday Prime Minister John Key said the Government would have backed a separate ACT bill through its first reading but the party chose to have their policy included in government legislation.
Mr Garrett told NZPA that ACT would reconsider its position if it failed to have the five-year threshold removed for strike offences.
"We would have to consider if the five year extra test that's currently there remained after negotiations with the National Party, we would then have to reconsider our position," he said.
"I don't want to go any further than that at this stage because there's a lot of water to go under the bridge. It's really premature to say we won't support it."
Rethinking Crime and Punishment director Kim Workman in April released information showing that of the 423 prisoners serving life sentences not one would have been stopped by the proposed three-strikes law.
Mr Garrett said as many as 77 lives would have been saved under the original ACT policy, but not under it as amended.
