The former Silver Fern defender took a year out from the job for family reasons, returning in 2014 to try and rebuild a young side. In her most recent stint the Mystics have had limited success.
They have won just two of their 10 outings so far this season, but are still in the mix to make the New Zealand conference play-offs with the top three progressing beyond the regular season.
Fuller said the team have their sights firmly set on post-season play again.
"One of the plus sides to having made the decision is that I can now just concentrate on the here-and-now. This playing group is dedicated to making the run home this season a successful one and that remains the key focus," she said.
The chair of the Northern Zone, Dianne McAteer, acknowledged Fuller's contribution over a long period.
"Debbie has been the most successful coach this franchise has seen and she's given enormously to netball in this region. We have big changes to deal with next season and by giving us the chance to make decisions early, Debbie has again acted with the 'team first' attitude she always has. We're sad to see her go, but respect her decision and wish her every success in the future."