"He's decided it's time to look for that next challenge in his career. We acknowledge the great work he has done here and wish him all the best for the future".
Preston described his time with the Highlanders as "five memorable years, with many highlights".
"I've learned a lot and enjoyed being part of the Highlanders family. It's been a pleasure to work with so many quality people, see the players' and the team's game grow and to be part of some pretty exciting football," Preston said in a statement.
"For me personally, it is important to keep developing and while I will miss the club, I now look forward to future opportunities".
The Highlanders will seek a replacement for Preston immediately and look to have the role filled as soon as possible.
Delaney will coach Canterbury through to the end of this year's National Provincial Championship, before relocating south to Otago.
He'll take over from Scott McLeod, who will assume an All Blacks assistant role after the imminent retirement of Wayne Smith.
Joe Maddock, meanwhile, will take over as Canterbury coach next year.