DIAGNOSIS Water is escaping from the brewing path. The usual suspects are a worn-out or mis-seated group head gasket (the rubber seal between the portafilter and the machine) or a leaking solenoid valve.
FIX First, check the simple stuff. Ensure your portafilter is absolutely clean and locked in tightly—turn it to the right until it's firmly seated at about a 30-degree angle, not just loosely hanging. Inspect the rubber group head gasket inside the group for cracks or debris. Clean it gently with a damp cloth. If it looks flattened or cracked, it needs replacement (Breville sells a kit).
IF THAT DIDN'T WORK If the leak persists with a clean, tight portafilter and a good gasket, the internal solenoid valve (which controls water flow) might have a failed seal. This is not a user-repairable part and requires a technician.
HEADS UP Never lock the portafilter in place with the machine on and no basket in it—high-pressure water can blow back and damage internal seals.