In a saucepan, add the peaches, mint leaves, sugar and water and bring to a boil.
Lower the heat a little, and stir the mixture with a wooden spoon for about 15 minutes, smashing the peaches, until it has become a thin syrup.
Add to a blender to puree it further if desired. Let it cool completely to room temperature before proceeding to the next step.
Remove the SCOBY and 1 cup of liquid from your gallon jar of kombucha. Pour the peach-mint syrup into the gallon. Do this slowly as it may foam up a little. Stir slowly with a wooden spoon.
Strain and bottle.
Leave about 1/2-1 inch of air at the top of the bottles and cover tightly. If you'd like extra fizz, let the bottles sit at room temperature for 1-3 days before storing in the refrigerator. Keep an eye on them to make sure they don't explode.
When ready to drink, remove form the fridge, strain out the newly formed SCOBY and enjoy!