Canterbury boat users must wear life jackets on vessels which are up to six metres long.
The Canterbury regional council says a fine could be imposed on anyone warned more than once, but the amount has yet to be decided.
The by-law was supposed to come into effect in April, but it was delayed by the February earthquake and became law on Wednesday.
Safety officers will be on the water enforcing the by-law when the boating season starts in summer.
Maritime New Zealand says it hopes to introduce the law nationally and a recommendation is being considered by the Government.