DIAGNOSIS That's the classic blender leak point. Food and liquid get forced under the sealing ring over time. If you don't clean it, it can mold and cause leaks.
FIX You need to remove the blade assembly from the jar. Use the provided wrench. Once it's off, you'll see the black rubber sealing ring. Carefully pry it out of its groove using a blunt tool like a butter knife or a fingernail. Wash the ring, the blade unit, and the groove in the jar with hot, soapy water. Let everything dry completely before reassembling.
IF THAT DIDN'T WORK If the rubber seal is cracked, stiff, or warped, cleaning won't restore it. You need a new one. Breville sells the 'Blade & Sealing Ring Assembly' as one part. It's easier and more hygienic to just replace the whole unit once a year or so if you use the blender heavily.
HEADS UP When you reassemble, make sure the rubber seal is seated perfectly in its groove before you screw the blade unit back on. A misaligned seal is the number one cause of new leaks after cleaning.