A u-turn from ACT, with the party now proposing a reversal of a National Government law it backed seven years ago.
The just-released election policy would make 17-year-olds be charged as adults when before the courts.
In 2016, National removed them from the adult justice system and put them into the youth justice framework.
ACT supported the move as part of the party's confidence and supply deal.
But party leader David Seymour says when something doesn't work, we should have the courage to change our minds and put it right.
"Letting 17-year-olds out of adult justice was a policy focused on the offender when we should be focused on victims."
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